title: Modify json field type via circe
body: <p>I have simple Json:</p>
  
  <pre><code>val str = """{"test":"123"}"""
  </code></pre>
  
  <p>How I can modify <code>String</code> <code>"123"</code> to <code>Int</code> <code>123</code> to get new Json?:</p>
  
  <pre><code>{"test":123}
  </code></pre>
  
  <p>Now I am using:</p>
  
  <pre><code>val json = parse(str).getOrElse(Json.Null)
  val jsObj = json.asObject.get  // Unsafe, just example
  val newJson = Json.fromJsonObject(jsObj.remove("test").add("test", Json.fromInt(123)))
  </code></pre>
  
  <p>But this code is not pretty.</p>
  
  <p>Is it possible to make this code prettier or maybe do it via circe optics?</p>