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/impl/basic_parser.ipp

@@ -48,6 +48,17 @@ template<bool isRequest>
 boost::optional<std::uint64_t>
 basic_parser<isRequest>::
 content_length() const
+{
+    BOOST_ASSERT(is_header_done());
+    if(! (f_ & flagContentLength))
+        return boost::none;
+    return len0_;
+}
+
+template<bool isRequest>
+boost::optional<std::uint64_t>
+basic_parser<isRequest>::
+content_length_remaining() const
 {
     BOOST_ASSERT(is_header_done());
     if(! (f_ & flagContentLength))
@@ -461,6 +472,7 @@ finish_header(error_code& ec, std::true_type)
     else
     {
         len_ = 0;
+        len0_ = 0;
         state_ = state::complete;
     }
 
@@ -796,6 +808,7 @@ do_field(field f,
 
         ec = {};
         len_ = v;
+        len0_ = v;
         f_ |= flagContentLength;
         return;
     }