&'a Headers does not allow reading Headers

[reem]

I've got a branch implementing a HeadersView that solves this problem, but right now that means you can't read the Headers of anything Streaming.

[seanmonstar]

Oh right, because it needs to be mutable internally. Maybe Headers could
keep a RefCell of the HashMap. It's mutability is just an implementation
detail, and shouldn't concern users.

That would also improve fighting the borrowck in matches.

[reem]

RefCell would not be a good solution here as it would be a leaky abstraction and lead to runtime failures when code tries to get two header references with overlapping lifetimes - this code would fail, for instance:

let length = req.headers.get_ref::<ContentLength>();
let encoding = req.headers.get_ref::<TransferEncoding>();

Right now this is a compile-time failure, using RefCell would make it a runtime failure.

We would also have to return cell::Ref and cell::RefMut instead of & and &mut.

[reem]

Basically scratch the above, I did crazy things with RWLock in #48 that fixes this and more.