Headers and MutableHeaders do not respect Mapping and MutableMapping interfaces

[adriangb]

For example, we have def keys(self) -> typing.List when it should be def keys(self) -> typing.KeysView[str] (KeysView acts like a set).

In {Immutable}MultiDict we solve this by having multi_items(self) -> typing.List[typing.Tuple[KeyType, ValueType]].

I feel that Headers and MutableHeaders should reflect what we do in ImmutableMultiDict and MultiDict. We could probably also do some more code sharing (have process_{keys,values} hooks in ImmutableMultiDict and MultiDict or something like that).

All this said, I'm not sure we could make these changes without it being a breaking change 😞