Don't send null values when updating batch

The Setra API does not accept null values for attributes, but it accepts
that the attribute is omitted. We should probably accept null values
for fields in order to avoid obscure bugs like this, but it does not
hurt to omit the values.

At least we use less of the bandwidth! :)

setra_client/client.py

@@ -383,7 +383,7 @@ class SetraClient(object):
         url = self.urls.put_update_batch()
         headers = {'Content-Type': 'application/json'}
         response = self.put(url,
-                            data=batchdata.json(),
+                            data=batchdata.json(exclude_unset=True),
                             headers=headers,
                             return_response=True)
         if response.status_code == 204: