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.

test/http/error.cpp

@@ -38,6 +38,7 @@ public:
     {
         check("http", error::end_of_stream);
         check("http", error::partial_message);
+        check("http", error::need_more);
         check("http", error::buffer_overflow);
         check("http", error::bad_line_ending);
         check("http", error::bad_method);