WP_REST_Request::remove_header() does not canonicalize header name

[TimothyBlynJacobs]

$request->add_header( 'Content-Type', 'application/json' );
$request->remove_header( 'Content-Type' );
$request->get_header( 'Content-Type' ); // still outputs 'application/json'

Patch incoming.

[joehoyle]

In 40577:

REST API: WP_REST_Request::remove_header() should canonicalize header names.

When headers are stored in WP_REST_Request internally they are canonicalized. This step already happens on setting / getting headers in any way, but was missed when implementing remove_header().

Props TimothyBlynJacobs.
Fixes #40347.