Change Body::size signature (API Change):

fix #497

This changes the function signature to accept the `value_type`
instead of the entire message.

Actions Required:

* For any user-defined models of Body, change the function signature
  to accept `value_type const&` and modify the function definition
  accordingly.

include/beast/http/impl/message.ipp

@@ -285,8 +285,7 @@ size() const
     static_assert(is_body_reader<Body>::value,
         "BodyReader requirements not met");
 
-    return size(detail::is_body_sized<
-        Body, decltype(*this)>{});
+    return size(detail::is_body_sized<Body>{});
 }
 
 template<bool isRequest, class Body, class Fields>