Request Headers 的其它 fields 值

來源：https://tools.ietf.org/html/rfc7231#section-5.3.4

例如：Accept-Encoding:gzip, deflate, sdch, br

一、Accept-Encoding：現實中，像郵局，將包裹如何，以及以哪一種方式打包。

    表示瀏覽器可接受tomcat 傳過來內容是如何編碼的，是否是個gzip壓縮格式，如java 檔案裡面的utf-8,Gb2312,unicode編碼等等，有利於瀏覽器可以解析傳過來的的檔案。1.0 沒有提供 q來選擇優先順序

The "Accept-Encoding" header field can be used by user agents to
   indicate what response content-codings (Section 3.1.2.1) are
   acceptable in the response.  An "identity" token is used as a synonym
   for "no encoding" in order to communicate when no encoding is
   preferred.

     Accept-Encoding  = #( codings [ weight ] )
     codings          = content-coding / "identity" / "*"

   Each codings value MAY be given an associated quality value
   representing the preference for that encoding, as defined in
   Section 5.3.1.  The asterisk "*" symbol in an Accept-Encoding field
   matches any available content-coding not explicitly listed in the
   header field.

   For example,

     Accept-Encoding: compress, gzip
     Accept-Encoding:
     Accept-Encoding: *
     Accept-Encoding: compress;q=0.5, gzip;q=1.0
     Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0

   A request without an Accept-Encoding header field implies that the
   user agent has no preferences regarding content-codings.  Although
   this allows the server to use any content-coding in a response, it
   does not imply that the user agent will be able to correctly process
   all encodings.

   A server tests whether a content-coding for a given representation is
   acceptable using these rules:

   1.  If no Accept-Encoding field is in the request, any content-coding
       is considered acceptable by the user agent.

   2.  If the representation has no content-coding, then it is
       acceptable by default unless specifically excluded by the
       Accept-Encoding field stating either "identity;q=0" or "*;q=0"
       without a more specific entry for "identity".

   3.  If the representation's content-coding is one of the
       content-codings listed in the Accept-Encoding field, then it is
       acceptable unless it is accompanied by a qvalue of 0.  (As
       defined in Section 5.3.1, a qvalue of 0 means "not acceptable".)

   4.  If multiple content-codings are acceptable, then the acceptable
       content-coding with the highest non-zero qvalue is preferred.

   An Accept-Encoding header field with a combined field-value that is
   empty implies that the user agent does not want any content-coding in
   response.  If an Accept-Encoding header field is present in a request
   and none of the available representations for the response have a
   content-coding that is listed as acceptable, the origin server SHOULD
   send a response without any content-coding.

      Note: Most HTTP/1.0 applications do not recognize or obey qvalues
      associated with content-codings.  This means that qvalues might
      not work and are not permitted with x-gzip or x-compress.i

二、Accept-Language：表示郵局的包裹要寄往那個國家，所在國家的交流語言。

      表示客戶瀏覽器優先使用什麼語言交流

    q:表示優先權

     例如：Accept-Language:zh-CN,zh;q=0.8

 Accept-Language = 1#( language-range [ weight ] )
     language-range  =
               <language-range, see [RFC4647], Section 2.1>

  Accept-Language: da, en-gb;q=0.8, en;q=0.7

would mean: "I prefer Danish, but will accept British English and other types of English".

      Cache-Control   = 1#cache-directive

     cache-directive = token [ "=" ( token / quoted-string ) ]

   For the cache directives defined below, no argument is defined (nor
   allowed) unless stated otherwise.

   to accept a response whose age is greater than the specified number of seconds. 

   The "max-age" 請求指令表示一個瀏覽器不願意接受一個響應的檔案，

這個響應檔案的age是大於指定秒數

   Unless the max-stale request directive is also present, 

   the client is not willing to accept a stale response.

   This directive uses the token form of the argument syntax: e.g.,
   'max-age=5' not 'max-age="5"'.  A sender SHOULD NOT generate the
   quoted-string form.

   Argument syntax:
      delta-seconds (see Section 1.2.1)

   The "max-stale" request directive indicates that the client is
   willing to accept a response that has exceeded its freshness
   lifetime.  If max-stale is assigned a value, then the client is
   willing to accept a response that has exceeded its freshness lifetime
   by no more than the specified number of seconds.  If no value is
   assigned to max-stale, then the client is willing to accept a stale
   response of any age.

   This directive uses the token form of the argument syntax: e.g.,
   'max-stale=10' not 'max-stale="10"'.  A sender SHOULD NOT generate
   the quoted-string form.

   Argument syntax:
      delta-seconds (see Section 1.2.1)   

   The "min-fresh" request directive indicates that the client is
   willing to accept a response whose freshness lifetime is no less than
   its current age plus the specified time in seconds.  That is, the
   client wants a response that will still be fresh for at least the
   specified number of seconds.

   This directive uses the token form of the argument syntax: e.g.,
   'min-fresh=20' not 'min-fresh="20"'.  A sender SHOULD NOT generate
   the quoted-string form.

   The "no-cache" request directive indicates that a cache MUST NOT use a stored response 

 to satisfy the request without successful validation on the origin server.

   請求指令表示瀏覽器不必使用cache 儲存一個響應，為了滿足瀏覽器請求，不需要在瀏覽器驗證成功，即瀏覽器緩
存版本校驗。

   a 504 (Gateway Timeout) status code.  If a group of caches is being
   operated as a unified system with good internal connectivity, a
   member cache MAY forward such a request within that group of caches.