Fix Bug #8 - Message posting with File URL isn't working

Treat files variable as an list (correct type) and ensure its contents
are utf8 strings by applying the utf8 function via map.

Author: cmlccie

ciscosparkapi/api/messages.py

@@ -236,7 +236,8 @@ def create(self, roomId=None, toPersonId=None, toPersonEmail=None,
         if markdown:
             post_data[u'markdown'] = utf8(markdown)
         if files:
-            post_data[u'files'] = utf8(files)
+            files = map(utf8, files)
+            post_data[u'files'] = files
         # API request
         json_obj = self.session.post('messages', json=post_data)
         # Return a Message object created from the response JSON data