Change hard-coded headernames to lowercase

[halvko]

The change at line 114 in src/security/mod.rs corrects a logic error, the rest is at best a small optimization and at worst a style choice

[halvko]

As I understand it, headers are not case agnostic for HTTP/2 connections “However, header field names MUST be converted to lowercase prior to their encoding in HTTP/2” (https://httpwg.org/specs/rfc7540.html#HttpHeaders).

[halvko]

And the documentation for ‘HeaderName::from_lowercase_str’ specifically states that the input is expected to be lowercase