unify use of 'std' prefix

Author: melode11

src/sio_client.h

@@ -56,7 +56,7 @@ namespace sio
         // Client Functions - such as send, etc.
         void connect(const std::string& uri);
 
-        void connect(const std::string& uri, const std::map<string,string>& query);
+        void connect(const std::string& uri, const std::map<std::string,std::string>& query);
 
         void set_reconnect_attempts(int attempts);