fix issue 787

Author: zhupumpkin

can/message.py

@@ -112,7 +112,7 @@ def __init__(self, timestamp=0.0, arbitration_id=0, is_extended_id=None,
         if is_extended_id is not None:
             self.is_extended_id = is_extended_id
         else:
-            self.is_extended_id = True if extended_id is None else extended_id
+            self.is_extended_id = False if extended_id is None else extended_id
 
         self.is_remote_frame = is_remote_frame
         self.is_error_frame = is_error_frame