title: Issue passing custom header values using rest-client-builder plugin
body: <p>I am using the latest grails (2.1.2) with rest-client-builder plugin version 1.0.3. I am trying to pass in a custom request header value (say SEC_USER) to the target server which pre-authenticates a request with this request header value. I am using the following code to pass the value in but I dont get the header value in the receiving end. </p>
  
  <pre><code>RestBuilder rest = new RestBuilder(connectTimeout:5000, readTimeout:20000); 
  rest.get("&lt;some uri&gt;") { 
      contentType MediaType.APPLICATION_JSON.toString() 
      header("SEC_USER", "foo") 
  } 
  </code></pre>
  
  <p>Looking at RequestCustomizer the header() method should set the HttpHeaders field as it calls 
  headers[name] = value </p>
  
  <p>Could this be an issue in the RestTemplate class (method exchange(...)) which this plugin uses internally? </p>