Compare Dictionary.Keys directly, without creating array

[XiaowenVC]

Resolve issue #25

[robertmogos]

Could you please update the test in SearchParametersTest so we can properly test this. Thanks

func testEquality() {
        let params1 = SearchParameters()
        params1.addFacetRefinement(name: "boo", value: "barry")
        params1.addFacetRefinement(name: "foo", value: "bar")
        let params2 = SearchParameters()
        params2.addFacetRefinement(name: "foo", value: "bar")
        params2.addFacetRefinement(name: "boo", value: "barry")
        XCTAssertEqual(params1, params2)
    }