fix issue leading to ~20x slowdown in binary json processing

Author: tomqext

rosbridge_library/src/rosbridge_library/protocol.py

@@ -127,7 +127,9 @@ def incoming(self, message_string=""):
         if self.bson_only_mode:
             self.buffer.extend(message_string)
         else:
-            self.buffer = self.buffer + str(message_string)
+            if isinstance(message_string, bytes):
+                message_string = message_string.decode('utf-8')
+            self.buffer = self.buffer + message_string
         msg = None
 
         # take care of having multiple JSON-objects in receiving buffer