Use to_ascii_lower() and eq_ignore_ascii_case() instead of unsafe code.

This reverts [some changes made during the 2014-03-04 Rust upgrade](https://github.com/mozilla/servo/commit/bbac8aa5c3d95e70151b824ca3443f0fb4d9b5a5#diff-d084c1d37003c83f26aa4dac0e8d9e6bL221). @larsberg, what was the reason for preferring the unsafe `.to_ascii_nocheck()`?

r? @larsberg