Simplify some type trait expressions

fix #466

example/http/server/fast/http_server_fast.cpp

@@ -88,7 +88,8 @@ public:
 
 private:
     using alloc_t = fields_alloc<char>;
-    using request_body_t = http::basic_dynamic_body<boost::beast::flat_static_buffer<1024 * 1024>>;
+    //using request_body_t = http::basic_dynamic_body<boost::beast::flat_static_buffer<1024 * 1024>>;
+    using request_body_t = http::string_body;
 
     // The acceptor used to listen for incoming connections.
     tcp::acceptor& acceptor_;