Description of problem:
In sat6.5 POST request to katello/api/v2/host_collections no longer returns host_ids field, which was present in 6.4 and below.

Version-Release number of selected component (if applicable):
Sat6.5 SNAP 5

How reproducible:
Always

Steps to Reproduce:
1. Make POST request to katello/api/v2/host_collections with some host ids, e.g.
{"name": "my_host_collection", "host_ids": [2], "organization_id": 3}
2. Take a look at received response
3.

Actual results:
No host ids in output:
{"name":"my_host_collection","organization_id":3,"max_hosts":null,"description":null,"total_hosts":1,"unlimited_hosts":true,"created_at":"2018-11-28 15:33:53 UTC","updated_at":"2018-11-28 15:33:53 UTC","id":9,"permissions":{"deletable":true,"editable":true}}

Expected results:
host_ids should be present in output, similar to GET:
{"host_ids": [2],"name":"my_host_collection","organization_id":3,"max_hosts":null,"description":null,"total_hosts":1,"unlimited_hosts":true,"created_at":"2018-11-28 15:33:53 UTC","updated_at":"2018-11-28 15:33:53 UTC","id":9,"permissions":{"deletable":true,"editable":true}}

Additional info: