Use of std::ascii::AsciiExt causes warnings

[kt3k]

std::ascii::AsciiExt has been deprecated since 1.26.0 and travis build shows warnings about them at compiling. See https://travis-ci.org/servo/rust-url/jobs/400867523 for example.

I think we should replace them with inherent method calls or at least suppress the warnings.

[nox]

Fixed in master.