async_write requires a non-const message:

This is necessary to meet the requirements for
asynchronous operations:

http://www.boost.org/doc/libs/1_64_0/doc/html/boost_asio/reference/asynchronous_operations.html

Specifically, that the caller must ensure the validity of the message
argument until the handler is invoked.

object

CHANGELOG.md

@@ -5,6 +5,8 @@ Version 64:
 * Simplify websocket write_op
 * Exemplars are compiled code
 * Better User-Agent in examples
+* async_write requires a non-const message
+* Doc tidying
 
 --------------------------------------------------------------------------------
 

include/beast/http/impl/write.ipp

@@ -359,7 +359,7 @@ class write_msg_op
             Body, Fields, no_chunk_decorator> sr;
 
         data(Handler& h, Stream& s_, message<
-                isRequest, Body, Fields> const& m_)
+                isRequest, Body, Fields>& m_)
             : s(s_)
             , sr(m_, no_chunk_decorator{})
         {
@@ -766,7 +766,7 @@ template<class AsyncWriteStream,
 async_return_type<
     WriteHandler, void(error_code)>
 async_write(AsyncWriteStream& stream,
-    message<isRequest, Body, Fields> const& msg,
+    message<isRequest, Body, Fields>& msg,
         WriteHandler&& handler)
 {
     static_assert(

include/beast/http/write.hpp

@@ -488,7 +488,7 @@ template<class AsyncWriteStream,
 async_return_type<
     WriteHandler, void(error_code)>
 async_write(AsyncWriteStream& stream,
-    message<isRequest, Body, Fields> const& msg,
+    message<isRequest, Body, Fields>& msg,
         WriteHandler&& handler);
 
 //------------------------------------------------------------------------------