Add alternative for HeaderMap::entry that cannot error

[seanmonstar]

Currently, HeaderMap::entry is generic over its argument, allowing lazily parsing a HeaderName, but means that some of those types might not be valid, so entry returns Result<Entry, Error>. When alreadying passing a HeaderName, it causes code to deal with an error that can never happen, like headers.entry(HOST).expect("Host is a valid name").

It'd be nice to add an alternative method that doesn't return a Result, but allows fewer types.

If considered for breaking changes, I'd rename the current to try_entry, and make the new alternative just entry.

[carllerche]

I agree with your proposed breaking change once users can define const header names (this year I think).

[seanmonstar]

Closed in #321 with a breaking change.