False positives header tests due to case sensitivity

Problem/Motivation

Since version 3.1.2 we're getting false positives as already described (and attempted to be fixed) in #3559063: X-Frame-Options false negative, but it's now created the issue instead of solving it.

The reason seems to be that curl responds with lower case header keys, while $this->httpClient->request('GET', $host)->getHeaders() responds with upper case header keys, at least in our instances. I don't know where the difference comes from, but the solution should be dealing with any scenario.

Proposed resolution

Do not just convert the tested header keys to lower case but also the keys in the tested array.