title: Codable enum with default case in Swift 4
body: <p>I have defined an <code>enum</code> as follows:</p>
  
  <pre><code>enum Type: String, Codable {
      case text = "text"
      case image = "image"
      case document = "document"
      case profile = "profile"
      case sign = "sign"
      case inputDate = "input_date"
      case inputText = "input_text"
      case inputNumber = "input_number"
      case inputOption = "input_option"
  
      case unknown
  }
  </code></pre>
  
  <p>that maps a JSON string property.
  The automatic serialization and deserialization works fine, but I found that if a different string is encountered, the deserialization fails. </p>
  
  <p>Is it possible to define an <code>unknown</code> case that maps any other available case?</p>
  
  <p>This can be very useful, since this data comes from a RESTFul API that, maybe, can change in the future.</p>