Improve Data, Date and URL coding

[gering]

This PR will change the default encoding strategies of Date, Data and URL objects, since they can be stored directly in Dictionaries unlike JSON, which does not support those types directly and thus defaults to formatted/base64 Strings.

• Data now does not encode to base64 String, but to NSData directly
• Data does decode from base64 String and NSData object
• Date objects do now encode to Date directly, if no other strategy is chosen
• Date objects do decode from all the variations but also from Date objects directly
• URLs now do not encode to Strings anymore, but to NSURL directly
• URLs do decode from Strings and NSURLs

Also, following test was not passing before, since a base64 encoded String was expected for Data objects.

    func testDecoding_dataFromVariousTypes_shouldSucceed() throws {
        // given
        struct JustData: Codable {
            let data: Data
        }

        let encoded1: [String: Any] = ["data": "dGVzdA=="]
        let encoded2: [String: Any] = ["data": "test2".data(using: .utf8)!]

        // when
        let decoded1 = try sut.decode(JustData.self, from: encoded1)
        let decoded2 = try sut.decode(JustData.self, from: encoded2)

        // then
        XCTAssertEqual(decoded1.data, "test".data(using: .utf8))
        XCTAssertEqual(decoded2.data, "test2".data(using: .utf8))
    }

This PR will also cleanup and add some more tests.

• Test function names follow now a common method naming convention
• Test functions are now grouped into "given, when, then" sections.