Increase code coverage

Currently, the unit tests cover around 53% of the code, I'm sure we can do better than that:

```console
$ mix test --cover
Cover compiling modules ...
.......**.........
Finished in 0.4 seconds (0.4s async, 0.00s sync)
18 tests, 0 failures, 2 skipped

Randomized with seed 451769

Generating cover results ...

Percentage | Module
-----------|--------------------------
0.00% | ExlasticSearch.Model.SearchResult
0.00% | ExlasticSearch.Retry.Decorator
0.00% | Scrivener.Paginater.ExlasticSearch.Query
22.00% | ExlasticSearch.Query
46.67% | ExlasticSearch.Aggregation
50.00% | ExlasticSearch.Retry.ExponentialBackoff
53.85% | ExlasticSearch.BulkOperation
60.00% | ExlasticSearch.Indexable.ExlasticSearch.MultiVersionTestModel
60.00% | ExlasticSearch.Indexable.ExlasticSearch.TestModel
60.00% | ExlasticSearch.Indexable.ExlasticSearch.TestModel2
63.22% | ExlasticSearch.Repo
66.67% | ExlasticSearch.Indexable
71.43% | ExlasticSearch.Model
71.43% | ExlasticSearch.Response
100.00% | ExlasticSearch
100.00% | ExlasticSearch.MultiVersionTestModel
100.00% | ExlasticSearch.MultiVersionTestModel.SearchResult
100.00% | ExlasticSearch.Response.Hits
100.00% | ExlasticSearch.Response.Record
100.00% | ExlasticSearch.Response.Search
100.00% | ExlasticSearch.RetryStrategy
100.00% | ExlasticSearch.TestModel
100.00% | ExlasticSearch.TestModel.SearchResult
100.00% | ExlasticSearch.TestModel2
100.00% | ExlasticSearch.TestModel2.SearchResult
100.00% | ExlasticSearch.TypeInference
100.00% | ExlasticSearch.TypeInference.API
100.00% | ExlasticSearch.TypeInference.Base
-----------|--------------------------
53.85% | Total

Coverage test failed, threshold not met:

Coverage: 53.85%
Threshold: 90.00%

Generated HTML coverage results in "cover" directory
```

Also, there are 2 unit tests that are currently skipped, we should see why those two tests are failing and fix the situation.