Rename `deleteHeader()` to `unsetHttpHeader()`

[ThomasLandauer]

To be consistent with all other header functions: haveHttpHeader() (to which it is the counterpart), seeHttpHeader(), seeHttpHeaderOnce(), etc.

Also in lib-innerbrowser

[ThomasLandauer]

Actually, "delete" isn't really the right term here. I'd say: Either unsetHttpHeader(), or (to be consistent with have...) dontHaveHttpHeader()

[SamMousa]

A PR to fix this could be implemented without breaking BC. Just add an alias for the "wrong" function name and mark it deprecated.
Move the implementation to the "right" function name.

[ThomasLandauer]

If you merge this, I'll do the same in lib-innerbrowser.