Make accept header comparison case-insensitive

[s-patompong]

This issue #39411 mentioned that the accepts method is currently not case-insensitive.

This pull request will cast both variables to lowercase before making the comparison. By doing so, the APPLICATION/JSON header can match application/json in the code.

The new test method is added to the \Illuminate\Tests\Http\HttpRequestTest class for case-insensitive test.

[crynobone]

The given RFC are for MIME. Are you certain that HTTP follows that? and why hasn't Symfony handle this already if that was the case?

[s-patompong]

My bad, I forgot to recheck the RFC link (copied it from the issue).

I'll find more info regarding the accept header.

[s-patompong]

It looks like the RFC doesn't mention the letter case requirement for the Accept field value at https://datatracker.ietf.org/doc/html/rfc7231#section-5.3.2.

However, it does state this below at https://datatracker.ietf.org/doc/html/rfc7231#section-5.3.1.

Many of the request header fields for proactive negotiation use a common parameter, named "q" (case-insensitive).

If q is case-insensitive, maybe it's worth it to see the other fields as case-insensitive as well?

[s-patompong]

Close this one since #39413 has more coverage :)