Fix / improve tests

[fschwahn]

Hi,
I addressed #65 and noticed that some tests are failing. The tests fail because nokogiri behavior changed slightly. Tests still pass with 1.6.6.2, and fail at least since 1.6.8.1 (I didn't check which exact nokogiri release changed this behavior because I think it doesn't matter.). What changed in Nokogiri was that multiple opening tags are no longer removed, but escaped. Example:

before:

full_sanitize("<<<a>") # => ""

after:

full_sanitize("<<<a>") # => "&lt;&lt;"

I adjusted the tests so they pass again.

Lastly, I fixed a deprecation warning from MiniTest.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @kaspth (or someone else) soon.

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[fschwahn]

CI failure is unrelated to this PR, it's because rubinius couldn't be installed. Maybe it should be added to allowed failures?

[kaspth]

Thanks!