Make the headers collection not throw on non-existent keys

[ZzZombo]

Problem Description

I do not understand why is it made so, it's just not convenient to have to wrap every access either in existence checks, error handlers, or use get_all and then do the concatenation yourself.

Proposal

Return None if the key doesn't exist.

Alternatives

See above.

Additional context

None.

[mhils]

As mentioned in the docs, headers mostly behave like a normal dict: You can call headers.get("key") or headers.get("key", "default") if you don't want them to raise.