title: extend Generic Method toJSON from jsonlite
body: <p>I'm writting a package and I want to extend the generic method <code>toJSON</code> included in the <code>jsonlite</code> package to handle one of my classes.
  I thought that I would be able by doing:</p>
  
  <p>1) Add jsonlite to the Depends in the DESCRIPTION file</p>
  
  <p>2) Take care about the Namespace by adding roxygen tags as</p>
  
  <pre><code>#' @importMethodsFrom jsonlite toJSON
  #' @export
  setMethod("toJSON", signature = "myClass", definition = function(x,...){
      jsonlite::toJSON(as.character(x))
  })
  </code></pre>
  
  <p>But when I try to build the package I get the following Error:</p>
  
  <blockquote>
    <p>Fehler in setGeneric(f, where = where) :    must supply a function
    skeleton for 'toJSON', explicitly or via an existing function</p>
  </blockquote>
  
  <p>In my understanding the roxygen <code>@importMethodsFrom jsonlite toJSON</code> tag should exactly do that, provide the generic...</p>