fix#884
* BodyReader and BodyWriter constructors now require the header and
body elements to be passed as distinct header and value_type objects.
This enables the composition of body types, for example:
http::response<compressed_body<http::string_body>> res;
* The previous single-argument constructors are deprecated and will be
removed in a subsequent version.
Actions Required:
* Change user-defined instances of BodyReader or BodyWriter constructor
signatures to the two-argument form.
OR
* Define the macro BOOST_BEAST_ALLOW_DEPRECATED in the project (which
will accept both the new and the deprecated signatures).
Body::reader and Body::writer meanings are reversed, for
consistency with the names of the stream operations:
* Body::reader is used for read, read_some, async_read, async_read_some
* Body::writer is used for write, write_some, async_write, async_write_some
Actions Required:
* Swap the reader and writer names for user defined Body types
* Swap use of is_body_reader and is_body_writer
