Handle types that are both RawRepresentable and JSONPrimitiveConvertible

[Jon889]

The use case is you can already have an enum that conform to JSONPrimitiveConvertible. But if you need the enum to have a raw type like String, then you get an ambiguous call because it's both RawRepresentable and JSONPrimitiveConvertable. So adding these methods resolves that ambiguity.

eg:

    public enum NotificationType: String, JSONPrimitiveConvertible {
	case info, warning, success, error
	public static func from(jsonValue: String) -> Self? {
		self.init(rawValue: jsonValue.trimmingWhitespace.lowercased())
	}
    }

    let x: NotificationType = properties.json(atKeyPath: "status") ?? .info

Now works with this PR.

Unfortunately function bodies are duplicated exactly from the JSONPrimitiveConvertable functions as I couldn't find a way to reference that exact method unambiguously.

[lucianomarisi]

@Jon889 could we have some unit tests for these new cases?