Content-Type must be written in lowercase

[cpeuschel]

In URequest.php method getContentType the header field "Content-Type" must be written in lowercase

See: https://tools.ietf.org/html/rfc7540#section-8.1.2

[jcheron]

Thank you for reporting it.
Indeed, it remained on the old RFC 7231 for HTTP/1.1
I have to make sure it doesn't create problems elsewhere before I change.