fix(headers): OrigHeaderMap constructor drops header casing

[tjaycodes4you]

Fix: OrigHeaderMap constructor drops header casing

Problem

OrigHeaderMap(list) does not preserve the casing of header names passed to it.

For example:

OrigHeaderMap(['Host', 'User-Agent', 'Accept'])

sends host, user-agent, accept on the wire — all lowercase — despite the title-case input.

Root cause

In src/header.rs, the new() constructor converts each string to a HeaderName via
HeaderName::from_bytes() before inserting it into the map:

let name = match name
    .extract::<PyBackedStr>()
    .map(|n| HeaderName::from_bytes(n.as_bytes()))  // normalises to lowercase
{
    Ok(Ok(n)) => n,
    _ => continue,
};
headers.insert(name);  // inserts as Kind::Standard → lowercase on wire

HeaderName is always stored lowercase (per the HTTP spec's normalisation). When inserted
into OrigHeaderMap, it becomes Kind::Standard, whose as_ref() returns the lowercase
canonical form — so the original casing is permanently lost.

By contrast, the insert() method and the FromPyObject impl both use Bytes::from_owner()
which goes through Kind::Cased, preserving the exact bytes as given.

Fix

Change new() to use the same Bytes path as insert() and FromPyObject:

let name = match name.extract::<PyBackedStr>() {
    Ok(n) => Bytes::from_owner(n),
    _ => continue,
};
headers.insert(name);  // inserts as Kind::Cased → exact casing preserved on wire

Impact

Any code that constructs OrigHeaderMap via the constructor (e.g. OrigHeaderMap(['Host', 'X-Custom']))
now correctly preserves the supplied casing. The insert() method was already correct and is unaffected.

[gemini-code-assist]

Code Review

This pull request simplifies the header name extraction logic in src/header.rs by directly converting PyBackedStr to Bytes. A suggestion was made to use a let-else statement instead of a match block to improve the readability of the extraction process.