Adds a @LossyOptional property wrapper

[moyerr]

When decoding an Optional type, it is still possible for decoding to fail and throw an error, rather than falling back to a nil value. This PR addresses that issue by adding a @LossyOptional property wrapper, which is just a typealias of @DefaultCodable using a new DefaultNilStrategy. Possible README usage description below:

@LossyOptional

Optional codable values can be frustrating in that, if decoding fails for any reason, we are often given an error instead of a nil value, like we might expect.

@LossyOptional fixes this annoyance by defaulting decoded Optionals to nil if the Decoder is unable to decode the value.

Usage

struct Image: Codable {
    @LossyOptional var url: URL?
}

let json = #"{ "url": "https://website .test" }"#.data(using: .utf8)!
let result = try JSONDecoder().decode(Image.self, from: json)

print(result.url) // nil

Without the @LossyOptional property wrapper here, we would get a DecodingError.dataCorrupted(_:) error instead.

[marksands]

I don't think @DefaultNil is a good name here, since this is really providing a catch handler for a failed decoding. I haven't come up with a good alternative though. @SetNilOnError, @FallbackToNil, @FailingToNil, etc. 🤔

[moyerr]

@marksands Okay yeah, I see what you mean. I like @FallbackToNil or @FailsToNil. Or what do you think about following the Lossy naming pattern and going with @LossyOptional? Whatever your preference, I'm happy to make the change 😄

[marksands]

I like LossyOptional. Let's run with that. That gives me some ideas to make some of my other implementations more generic and flexible. 🙂

It also might be worth adding a test that demonstrates the Lossy problem outside of URL types:

	func testDecodingWithBadIntegerConversion() throws {
		struct Fixture: Codable {
			@LossyOptional var a: Int?
			@LossyOptional var b: Int?
		}

		let jsonData = #"{ "a": 3.14, "b": 3 }"#.data(using: .utf8)!
		let fixture = try JSONDecoder().decode(Fixture.self, from: jsonData)
		XCTAssertNil(fixture.a)
		XCTAssertEqual(fixture.b, 3)
	}

[marksands]

See also the comment I left about adding an additional fixture with another type(s).

[moyerr]

@marksands Thanks for the feedback! Changes addressed, and original PR description updated with name change 😃

[marksands]

Thank you! 🙇