Ensure `mongodb` is correctly supported

[brunoocasali]

This gem has basically no MongoId test cases. We need to add tests and guarantee mongo is being correctly handled.

about reported bugs:

• problem support custom primary key #251
• can't search with mongoid #250

about tests:

We could start thinking of a way to reuse the same use cases between multiple integrations.

eg:

• context 'when using with mongoid'
  • indexes correctly the data
  • searches through the indexed data
• context 'when using with active_record'
  • indexes correctly the data
  • searches through the indexed data
• context 'when using with sequel'
  • indexes correctly the data
  • searches through the indexed data

each context here could be a different directory, and the it steps can be shared_examples.

[brunoocasali]

I assume this is not a good first issue because it could be quite complex, but I would like to have some community help here if we start small ✋.

[brunoocasali]

Also mentioned here: #250 and #251. This gem is not doing a great job of supporting mongo db environments.

This issue started as a testing enhancement but should be more than that because today, it is not possible to confirm if we support or not mongo environments.