Header fields should be case insensitive (RFC 7230 §3.2, RFC 9110 §5.1)

I'm working with http::HeaderMap and noticed that HeaderMap::get, HeaderMap::insert, and HeaderName::from_static all seem to be case sensitive (in fact, the latter panics:

HeaderName::from_static("Content-Type")

panics with:

index out of bounds: the len is 0 but the index is 0

while:

HeaderName::from_static("content-type")

does not.

This goes against the HTTP spec requiring case-insensitive handling of header values:

• RFC 7230 §3.2

  Each header field consists of a case-insensitive field name followed by a colon (":"), optional leading whitespace, the field value, and optional trailing whitespace.

• RFC 9110 §5.1

  Field names are case-insensitive...

IMO, this is distinct from #670, and in fact case-insensitive keys should solve that issue as well, while not requiring the original cases to be changed.

It looks like crates like case_insensitive_hashmap and unicase are examples of case-insensitive String handling.

Is there any way to just make the HeaderMap case-insensitive for retrieval? That would preserve original case as well as allow any case for HeaderMap::get to meet the spec and prevent errors.

Start by examining the HeaderMap::get and HeaderMap::insert behavior together with HeaderName::from_static, using the case-sensitive examples in the issue as the reproduction. Done means header-name retrieval accepts differing capitalization, from_static no longer panics for valid field names, and original casing remains preserved.