Test the behaviors (HTTP client) implementation

[ivanhercaz]

This issue is just a reminder of what it is pending to do after the merge of #21 and the development of the HTTP client of ExTldr with the HTTPoison behaviour described in #22.

When the behavior of HTTPoison will be integrated in ExTldr it will be useful to develop the tests to cover the working of these lines of code. I label this issue as priority:low because it is important but not explicitly necessary to the good working of ExTldr, it is more an idea to keep the code well-documented and clean, easing the future participation of possible collaborators.

Before to work on this it is important to check the test already writtent, but commented, in test/ex_tldr_test.exs, review again #21 and check the saved readings about mocking in Elixir, behaviors and a long etcetera of useful stuff for this issue.