Deprecate `Download` shorthand header.

We should strive for as little magicness as possible. But when
we provide convenience methods they should entirely cover the case.

With forcing content download there's already some additional preparation to do
which we also didn't catch here i.e. setting Content-Type to application/octet-stream.

The magic Download header was never supported when passing headers as an array.

Updating tests; moving location test into other existing method.