Http.responseHeaders should be case insensitive

as per standard http/1 response headers are case insensitive. The current implementation as `Map` is case sensitive and leads to wrong behavior. I was just bitten by an upgrade of a proxy server which now rewrites all headers to lower-case (like http/2) as default. I guess this will happen more and more, therefore I suggest to replace responseHeaders with a case insensitive implementation (convert all values to lower case).