Make \Illuminate\Testing\TestResponse::assertHeader() case insensitive

[HenkPoley]

I bumped into a PhpUnit error today when asserting the response has the correct header:

$response->assertHeader('content-type', 'text/html; charset=UTF-8');

PhpUnit error:

Header [content-type] was found, but value [text/html; charset=utf-8] does not match [text/html; charset=UTF-8].
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'text/html; charset=UTF-8'
+'text/html; charset=utf-8'

[..paths omitted..]

It might be that redirect('/some/path') fairly recently changed from UTF-8 to utf-8 due to HPACK/QPACK normalisation rules. But it broke some of my tests.

In principle these HTTP headers are case-insensitive, so this change to \Illuminate\Testing\TestResponse::assertHeader() makes sense.

Let me know if you agree, or you want it fixed in another way.

[HenkPoley]

Ah.. I see this message appear elsewhere:

Thank you for your pull request. However, you have submitted this PR on the Illuminate organization which is a read-only sub split of laravel/framework. Please submit your PR on the https://github.com/laravel/framework repository.

Thanks!

I'll check over there.

[HenkPoley]

Proper pull request here: laravel/framework#58383