Discovery create_collection must send JSON

This fixes the problem described here:
https://stackoverflow.com/questions/44707586/watson-discovery-service-create-collection-api-call-returns-error-415-unsuppo

Author: bruceadams

watson_developer_cloud/discovery_v1.py

@@ -222,7 +222,7 @@ def create_collection(self,
             environment_id)
         return self.request(method='POST',
                             url=url_string,
-                            data=data_dict,
+                            json=data_dict,
                             params={'version': self.version},
                             accept_json=True)