Add test for #507

tests/functional/001_records/078_edit_event_with_nullable_bigint.log

 Content-Length: 56
 
 {"id":1,"name":"Launch","datetime":null,"visitors":null}
+===
+PUT /records/events/1
+
+{"datetime":"2016-01-01 13:01:01","visitors":0}
+===
+200
+Content-Type: application/json
+Content-Length: 1
+
+1
+===
+GET /records/events/1
+===
+200
+Content-Type: application/json
+Content-Length: 70
+
+{"id":1,"name":"Launch","datetime":"2016-01-01 13:01:01","visitors":0}