oauth: handle a string expires_in value gracefully

If an OAuth token's expires_in field is a string instead of an int,
simply try to convert it to an int first.

Fixes #179

fhirclient/auth.py

@@ -283,7 +283,7 @@ def _request_access_token(self, server, params):
         del ret_params['access_token']
 
         if 'expires_in' in ret_params:
-            expires_in = ret_params.get('expires_in')
+            expires_in = int(ret_params['expires_in'])
             self.expires_at = datetime.now() + timedelta(seconds=expires_in)
             del ret_params['expires_in']