[FIX] Merge returned `fields` with `_source` if both are returned

Previously, when asking for `_source` in `fields`, only the specific `fields`
have been returned, not the full _source.

The Elasticsearch behaviour has possibly changed here,
see <https://github.com/karmi/tire/pull/31#issuecomment-1340967>

Closes #663, closes #687, closes #712
[TEST] Added a unit test for using `_source` in conjunction with `fields`

Related: #663, #687 #712
[TEST] Added an integration test for fields / script fields / _source

Related: #663, #687 #712