Different heading formation in GET and HEAD

[cthulhu-rider]

Seems like GET and HEAD handlers form identity headers differently.

[KirillovDenis]

I'm not sure this is a bug because r.Response.Header.Set function normalize header key (if disableNormalizing isn't set). But I agree we should use the same keys (probable we should define constants for that)

[KirillovDenis]

It turns out we disabled normalization.

I suppose the reason is reverse proxy:

Disabled header names' normalization may be useful only for proxying
incoming requests to other servers expecting case-sensitive
header names. See valyala/fasthttp#57
for details.