title: When I use ObjectMapper in my code, I get the issues that "Left side of mutating operator has immutable type"
body: <p>How can I fix it?
  I just convert my app to Swift 4.2 which used Swift 3.0.
  And I use the ObjectMapper(3.4.2).</p>
  
  <pre><code>class Discuess&lt;author:PostAuthor,reply_to:Reply_To&gt;: Mappable {
  
       var author: PostAuthor? = nil
       var reply_to: Reply_To? = nil
        required init?(map: Map) {
  
  }
  func mapping(map: Map) {
      author &lt;- map["author"] //Left side of mutating operator has immutable type 'author.Type'
      reply_to &lt;- map["reply_to"] //Left side of mutating operator has immutable type 'reply_to.Type'
       }    
  }
  </code></pre>