use HeaderName::from_static in Send::check_headers

[srijs]

Prevents parsing the header names (due to the AsHeaderName impl on &str) over and over again at runtime. It's just a small optimization in my case (sub-1% cpu time) but given the simplicity of the change seemed worth making in any case.

[srijs]

@hawkw thanks for the quick approve! I updated the branch based on the failed build, but looks like this needs another approval to re-run the tests?

[hawkw]

re-approved, hopefully everything passes this time!