Fix and test serialization

[SimonSapin]

The serialization code (that I’m about to submit in a pull request) is incorrect and fails its tests, which are for now disabled. But it is good enough for the purpose of error reporting in Servo.

I believe that the spec is incorrect: http://dev.w3.org/csswg/css-syntax/#serialization

• Fix the spec
• Update the implementation accordingly
• Make sure the tests pass
• Add more tests that trigger serialization corner cases

[SimonSapin]

The existing tests pass. I don’t really want to bother adding more, it’s such uninteresting corner cases…