Added Goutte\Client->resetAuth()
feature #121 Added Goutte\Client->resetAuth() (elazar)

This PR was squashed before being merged into the master branch (closes #121).

Discussion
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Added Goutte\Client->resetAuth()

Once `setAuth()` is called on an instance of `Goutte\Client`, it's not possible to clear the HTTP authentication credentials. This pull request adds this feature. A use case for it is the ability to reuse the same client object between [Codeception](http://codeception.com/) tests, which is currently only possible by subclassing `Goutte\Client`.

Commits
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d4e40f1 Added Goutte\Client->resetAuth()