Mapping: Posting a mapping with default analyzer fails

Closes #2716

When merging two mappings, default index analyzers are represented by either null or by a "default"-named index analyzer object. Fixed a spot where only the null representation was being considered as default.

Wrote a simple REST test to confirm the behavior is fixed. All tests pass.
Could we maybe check the resulting mapping is as expected by doing a get_mapping call rather than assuming that no exception means everything is as expected?