code: public Object call(Map m) throws IOException {
          writer.write(JsonOutput.toJson(m));
  
          return m;
      }
docs: Named arguments can be passed to the JSON builder instance to create a root JSON object
  <p>
  Example:
  <pre class="groovyTestCase">
  new StringWriter().with { w ->
  def json = new groovy.json.StreamingJsonBuilder(w)
  json name: "Tim", age: 31
  
  assert w.toString() == '{"name":"Tim","age":31}'
  }
  </pre>
  
  @param m a map of key / value pairs
  @return a map of key / value pairs