This makes the `u16` in `Quality` private, since it only has a valid
range of 0-1000, and can't be enforced in public. The `q` function now
allows both `f32`s and `u16`s to construct a `Quality`.
BREAKING CHANGE: Any use of `Quality(num)` should change to `q(num)`.
The `TE` header is used by a client to specify which
transfer encodings other than `chunked` it will accept.
It also specifies whether HTTP trailers are acceptable.
This commit also adds a `Trailers` variant to the
`hyper::header::shared::Encoding` enum.
Closes#1109
BREAKING CHANGE: The `Encoding` enum has an additional
variant, `Trailers`.