title: Http4s EntityDecoder not being auto derived for simple case class
body: <p>I am getting this error:</p>
  
  <pre><code>Cannot decode into a value of type com.blah.rest.model.UserProfile, 
  because no EntityDecoder[cats.effect.IO, com.blah.rest.model.UserProfile] 
  instance could be found.
  </code></pre>
  
  <p>for the following case class:</p>
  
  <pre><code>case class UserProfile(id: Option[Int], firstName: String, lastName: String)
  </code></pre>
  
  <p>Encountered the error on <code>POST</code> code:</p>
  
  <pre><code>case req @ POST -&gt; Root / "v1" / "profiles" =&gt;
    req.as[UserProfile] flatMap {(up: UserProfile) =&gt;
      Ok(service.createProfile(up).asJson)
    }
  </code></pre>
  
  <p>With the following <code>POST</code> body:</p>
  
  <pre><code>{
     "firstName": "Jack",
     "lastName": "Appleseed"
  }
  </code></pre>
  
  <p>I think this happens when the body is being converted to <code>UserProfile</code> in <code>req.as[UserProfile]</code>!</p>
  
  <p>But, this is a plain vanilla case class, the <code>EntityDecoder</code> should be auto-derived! I know <code>akka-http</code> does it!</p>
  
  <p>Any ideas/suggestions?</p>
  
  <p>Please Note:   <code>Http4sVersion  = "0.18.0-M4"</code> and <code>circe version "0.9.0-M1"</code></p>