Changing U() macro to _XPLATSTR() in public headers

Helps avoid conflicts if _TURN_OFF_PLATFORM_STRING is defined

Author: moozzyk

.gitignore

@@ -19,3 +19,5 @@ artifacts/
 src/signalrclientdll/version.h
 /.vs/
 build.*
+signalrclient.VC.VC.opendb
+signalrclient.VC.db

include/signalrclient/connection.h

@@ -21,7 +21,7 @@ namespace signalr
     public:
         typedef std::function<void __cdecl(const utility::string_t&)> message_received_handler;
 
-        SIGNALRCLIENT_API explicit connection(const utility::string_t& url, const utility::string_t& query_string = U(""),
+        SIGNALRCLIENT_API explicit connection(const utility::string_t& url, const utility::string_t& query_string = _XPLATSTR(""),
             trace_level trace_level = trace_level::all, std::shared_ptr<log_writer> log_writer = nullptr);
 
         SIGNALRCLIENT_API ~connection();