Increase code coverage

[milmazz]

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

$ 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.