Fixes to rfc7230:

fix #1435, fix #1438

* Example and test can be built on msvc v141 15.9.6
  using /std:c++17 and BOOST_BEAST_USE_STD_STRING_VIEW.

* changed string_view.to_string() to std:string(string_view),
  awaiting availability of Library Fundamentals TS here.

* Reactivated relevant tests to param_list. #ifdef 0 test
  exhibited same assertion failed error in debug mode.
  Now fixed in DEBUG on msvc v141 15.9.6 with
  BOOST_BEAST_USE_STD_STRING_VIEW and /std:c++17.

* Looked up http paramters (transfer-encoding, etc.)
  and changed tests as well as fixing comment to
  match specs.

example/http/server/small/http_server_small.cpp

@@ -119,7 +119,7 @@ private:
             response_.set(http::field::content_type, "text/plain");
             beast::ostream(response_.body())
                 << "Invalid request-method '"
-                << request_.method_string().to_string()
+                << std::string(request_.method_string())
                 << "'";
             break;
         }