Add testcase #2025 for document.getElementById().
Make Attr::SetValue uses the internal setter.
Make Attr::SetValue to reflect attribute change.
Make Attr handles all setting value.
auto merge of #2110 : saneyuki/servo/attr, r=Ms2ger

- Fix: https://github.com/mozilla/servo/issues/2025
- Close: https://github.com/mozilla/servo/issues/2077