fix small bug with extensions

Signed-off-by: ryandawsonuk <rd@seldon.io>

cloudevents/sdk/event/base.py

@@ -154,7 +154,7 @@ def MarshalBinary(
                 if value is not None:
                     headers["ce-{0}".format(key)] = value
 
-        for key, value in props.get("extensions"):
+        for key, value in props.get("extensions").items():
             headers["ce-{0}".format(key)] = value
 
         data, _ = self.Get("data")