Headers are not handled in a case-insensitive way

The check for the "X-elastic-product" header is currently case sensitive. However http response header names are
supposed to be treated as case-insensitive (https://datatracker.ietf.org/doc/html/rfc7230#section-3.2). The impact is that if header names are lowercased by a proxy then elasticsearch hadoop will not connect with elasticsearch for any version at 7.14 or higher.