Improve the coverage metrics by improving* tests

[castarco]

Test annotations

Currently, the unit tests of the library aren't properly marked with the @uses and @covers annotations to indicate what's the test intending to cover. This fact leads us to an optimistic and erroneous code coverage ratio, potentially hiding important bugs.

Using Mutation testing, at least locally

Mutation testing it's very useful to discover new bugs and discover test coverage "holes". Recently has appeared a new PHP mutation testing called Humbug, which could be used to improve the library's tests.

I've been experimenting without success (I didn't spent enough time, and the project is currently in alpha or beta state), but if anyone wants to experiment, I think the task is worth.

• [ ] Tests annotations
• [ ] Mutation testing