HTTP improvements:

* Increased code coverage

* New Body concepts:
    is_Body
    is_ReadableBody
    is_WritableBody

include/beast/websocket/impl/accept_op.ipp

@@ -33,7 +33,7 @@ class stream<NextLayer>::accept_op
     struct data
     {
         stream<NextLayer>& ws;
-        http::request_v1<http::empty_body> req;
+        http::request_v1<http::string_body> req;
         Handler h;
         bool cont;
         int state = 0;

include/beast/websocket/impl/stream.ipp

@@ -253,7 +253,7 @@ accept(ConstBufferSequence const& buffers, error_code& ec)
     stream_.buffer().commit(buffer_copy(
         stream_.buffer().prepare(
             buffer_size(buffers)), buffers));
-    http::request_v1<http::empty_body> m;
+    http::request_v1<http::string_body> m;
     http::read(next_layer(), stream_.buffer(), m, ec);
     if(ec)
         return;