Added header function to extract specific value

[hanscj1]

Added a function that supports UniCase and extracts the specific value for a given header.

[seanmonstar]

Thanks for the pull request!

Is there a reason to include this in this crate, instead of just implementing this where one needs it? An issue I see here is that the unicase crate needs std, and httparse tries to work without it.

[hanscj1]

I'll take a look at doing that in my library. It was just easier at the time. thx

[hanscj1]

Just curious, I have seen several libraries lately working toward !std, why? thx

[seanmonstar]

I don't have any special interest in bending over backwards to make it work with no_std, but httparse just didn't need anything in std. The benefit is that people can use those libraries on small hardware, where executable size may be important, or access to a heap may not even exist.