title: Using a custom ISO8601DateFormatter with JSONEncoder
body: <p>I have the following date formatter:</p>
  <pre><code>let formatter = ISO8601DateFormatter()
  formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
  </code></pre>
  <p>I want to pass that into a <code>JSONEncoder</code>, like this:</p>
  <pre><code>let encoder = JSONEncoder()
  encoder.dateEncodingStrategy = .formatted(formatter)
  </code></pre>
  <p>but that produces an error as it only accepts custom <code>DateFormatters</code> not <code>ISO8601DateFormatters</code>.</p>
  <p>How can I do this?</p>