Need to keep up with std::ascii reform

[akosthekiss]

When building cargo with (very) recent rustc, the following error is reported:

/home/akiss/.cargo/registry/src/github.com-1ecc6299db9ec823/curl-0.1.3/src/ffi/easy.rs:153:37: 153:55 error: type `collections::string::String` does not implement any method in scope named `into_ascii_lower`
/home/akiss/.cargo/registry/src/github.com-1ecc6299db9ec823/curl-0.1.3/src/ffi/easy.rs:153         let name = name.to_string().into_ascii_lower();

If I'm right, this is since rust-lang/rust@070ab63 , and the cause of the error is also present in current HEAD of curl-rust.
(PS: I know that cargo officially does not necessarily build with latest rustc, but both cargo and curl will eventually run into this issue.)