fix#778
* The body data member is replaced with accessor
member functions.
Actions Required:
* Call member function message::body instead of accessing
the data member at call sites.
* HTTP read and write operations now return the
number of bytes transferred to or from the stream.
Actions Required:
* Modify HTTP read and/or write completion handlers to receive
the extra std::size_t bytes_transferred parameter.
* Stream write operations now return the number of bytes
transferred from the caller's input buffers.
Actions Required:
* Modify websocket write completion handlers to receive
the extra std::size_t bytes_transferred parameter.
Vinnie Falco