Remove unnecessary generic parameter from Set{Request,Response}HeaderLayer

[jplatte]

Motivation

Currently, SetRequestHeaderLayer and SetResponseHeaderLayer have to have a turbofish supplied when used with a plain HeaderValue (or Option<HeaderValue>) instead of a closure, which is annoying.

Solution

Adjust the generics to make things work out :)

Unfortunately, this conflicts with #147 and as such error messages from closures with wrong parameter types will continue to be reported rather late somewhere in the usage of these layers rather than at the construction site. However, I consider this change to be more valuable than #147.

This is a breaking change because it changes the number of generic parameters and previous ::<_, Body> turbofish usage will thus become invalid.

Properly fixes #127

[davidpdrsn]

I think this looks good and is worth doing. I've seen several people be confused about the turbofish. Wanna rebase/merge the lastest master and add a note to the changelog? Then it'll be gtm.

[jplatte]

Done.

[davidpdrsn]

Thanks!