swift Dictionary subscript with String enum

Dictionary+Enum.swift

protocol StringEnum {
    var rawValue: String { get }
}

extension Dictionary {
    subscript(enumKey: StringEnum) -> Value? {
        get {
            if let key = enumKey.rawValue as? Key {
                return self[key]
            }
            return nil
        }
        set {
            if let key = enumKey.rawValue as? Key {
                self[key] = newValue
            }
        }
    }
}

//Usage:
enum JSONKey: String {
    case identifier = "id"
    case token
    case firstName = "first_name"
    case lastName = "last_name"
}

extension JSONKey: StringEnum { }

let userDict: [String: AnyObject] = [
    "first_name": "Edward",
    "last_name": "Gorey",
    "token": "sdfkjhadfgq3894tq34tq34gq3489g",
    "id": "293598346798",
]

let firstName = userDict[JSONKey.firstName]
let lastName = userDict[JSONKey.lastName]

var newUser = userDict

newUser[JSONKey.token] = "different token"
newUser[JSONKey.lastName] = "Cullen"
newUser[JSONKey.identifier] = "666"