gzipAssertion = function () {
        confirmCompression.call(this, 'gzip');
    }
Checks that the response is gzip compressed
    @alias module:chakram-expectation.gzip
    @example 
    it("should detect gzip compression", function () {
        var gzip = chakram.get("http://httpbin.org/gzip");
        return expect(gzip).to.be.encoded.with.gzip;
    });