fix: case insensitive headers

[FFawzy]

The current HTTP reference stats that the headers should be case insensitive https://www.rfc-editor.org/rfc/rfc9112#name-field-syntax “Each field line consists of a case-insensitive field name followed by a colon (“:”)”

but the current implementation was only allowing headers that start with a capital letter. we faced the issue for example while using Google Cloud Run that it automatically lowercase all headers therefore the plugin does not work as well.

this PR is introducing a fix that makes the headers case insensitive as the HTTP reference suggests

fixes #18