Refactor HTTP serialization (API Change):

A new class `serializer` is introduced to permit incremental
serialization of HTTP messages. Existing free functions are
re-implemented in terms of this new class.

* The BodyReader concept is refined to support a greater variety
of strategies for providing buffers representing the body to
the serialization algorithms.

* Added buffer_body, a new model of Body which allows the caller
to provide a series of owned buffers using their own serialization
loop.

* Added empty_body, a model of Body which is for serialization only,
to represent HTTP messages with no content body.

* Removed overloads of write and async_write which send only
the HTTP header.

* Removed public interfaces for performing low-level chunk encoding.

extras/beast/test/string_iostream.hpp

@@ -114,9 +114,9 @@ public:
     write_some(
         ConstBufferSequence const& buffers, error_code&)
     {
-        auto const n = buffer_size(buffers);
         using boost::asio::buffer_size;
         using boost::asio::buffer_cast;
+        auto const n = buffer_size(buffers);
         str.reserve(str.size() + n);
         for(auto const& buffer : buffers)
             str.append(buffer_cast<char const*>(buffer),