Post new batches without sending None-fields

Exclude unset values from beeing transfered when posting a new batch to
Setra. Setra does not accept fields set to None (at least period and
vouchertype).

setra_client/client.py

@@ -285,7 +285,7 @@ class SetraClient(object):
         url = self.urls.post_new_batch()
         headers = {'Content-Type': 'application/json'}
         response = self.post(url,
-                             data=batchdata.json(),
+                             data=batchdata.json(exclude_unset=True),
                              headers=headers,
                              return_response=True)
         try: