Fixes #30, serializable issue with attachments

groupy/api/endpoint.py

@@ -479,13 +479,20 @@ def post(cls, bot_id, text, *attachments, picture_url=None):
         :returns: the created message
         :rtype: :class:`dict`
         """
+        def serializable(attachments):
+            for attachment in attachments:
+                try:
+                    yield attachment.as_dict()
+                except AttributeError:
+                    yield attachment
+
         r = requests.post(
             cls.build_url('post'),
             data=json.dumps({
                 'bot_id': bot_id,
                 'text': text,
                 'picture_url': picture_url,
-                'attachments': attachments
+                'attachments': list(serializable(attachments)),
             }),
             headers={'content-type': 'application/json'}
         )