basic_parser::content_length is stable (API Change):

fix #1375

* The value returned from `basic_parser::content_length`
  no longer changes as the body of the message is received.

Actions Required:

* Call `basic_parser::content_length_remaining` instead
  of `basic_parser::content_length`.

include/boost/beast/http/basic_parser.hpp

@@ -72,6 +72,7 @@ class basic_parser
     std::uint64_t body_limit_ =
         default_body_limit(is_request{});   // max payload body
     std::uint64_t len_ = 0;                 // size of chunk or body
+    std::uint64_t len0_ = 0;                // content length if known
     std::unique_ptr<char[]> buf_;           // temp storage
     std::size_t buf_len_ = 0;               // size of buf_
     std::size_t skip_ = 0;                  // resume search here
@@ -231,6 +232,18 @@ public:
     boost::optional<std::uint64_t>
     content_length() const;
 
+    /** Returns the remaining content length if known
+
+        If the message header specifies a Content-Length,
+        the return value will be the number of bytes remaining
+        in the payload body have not yet been parsed.
+
+        @note The return value is undefined unless
+              @ref is_header_done would return `true`.
+    */
+    boost::optional<std::uint64_t>
+    content_length_remaining() const;
+
     /** Returns `true` if the message semantics require an end of file.
 
         Depending on the contents of the header, the parser may