to_ascii_lower -> String#into_ascii_lower to match Rust nightly

I am unable to compile mime against Rust nightly, I get:

```
src/encoding/label.rs:7:5: 7:28 error: unresolved import `std::ascii::StrAsciiExt`. There is no `StrAsciiExt` in `std::ascii`
src/encoding/label.rs:7 use std::ascii::StrAsciiExt;
                            ^~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
Build failed, waiting for other jobs to finish...
/Users/john/.cargo/git/checkouts/mime.rs-bb2736419bbdb48d/master/src/lib.rs:28:5: 28:28 error: unresolved import `std::ascii::StrAsciiExt`. There is no `StrAsciiExt` in `std::ascii`
/Users/john/.cargo/git/checkouts/mime.rs-bb2736419bbdb48d/master/src/lib.rs:28 use std::ascii::StrAsciiExt;
                                                                                   ^~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
Could not compile `encoding`.
```

This is a quick patch to update the use of to_ascii_lower to use into_ascii_lower on String
I'd use `std::ascii::AsciiExt` here.