Response isJson() method does not work for lowercased headers

[Riley19280]

I am working with the FastAPI python server, that serves the headers in lowercase according the the HTTP RFC, mentioned here.

According to RFC for HTTP1.x in any case headers key must be case insensitive and a common implementation for performance reason is to lowercase it.
According to RFC for HTTP2.x headers key must be case insensitive and must be lowercase, so it gives more sense for an implementation to workwith lowercase. In HTTP 2.x not lowercase key should be dropped or refused.

the isJson method expects it to be cased as Content-Type, not content-type, and will incorrectly return false if a lowercase