title: Failing to set the user agent via httr::user_agent
body: <p>Is there anything special I need to consider when trying to change the user agent via <code>httr::user_agent</code> in a <code>httr::GET()</code> call on MS Windows? I'm using <code>R-3.1.0</code> and <code>httr 0.3</code>.</p>
  
  <p>Following the example at <code>?user_agent</code>, I'm getting these results:</p>
  
  <pre><code>url_this &lt;- "http://httpbin.org/user-agent"
  </code></pre>
  
  <p>Standard user agent:</p>
  
  <pre><code>GET(url_this)   
  Response [http://httpbin.org/user-agent]
    Status: 200
    Content-type: application/json
  {
    "user-agent": "curl/7.19.6 Rcurl/1.95.4.1 httr/0.3"
  } 
  </code></pre>
  
  <p>Modified user agent:</p>
  
  <pre><code>GET(url_this, user_agent("Mozilla/5.0"))
  Response [http://httpbin.org/user-agent]
    Status: 200
    Content-type: application/json
  {
    "user-agent": "curl/7.19.6 Rcurl/1.95.4.1 httr/0.3"
  }
  </code></pre>
  
  <p>I had expected that the second call returns something closer to what I'm getting when visiting <code>url_this</code> in my browser:</p>
  
  <pre><code>{
    "user-agent": "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:29.0) Gecko/20100101 Firefox/29.0"
  }
  </code></pre>
  
  <p>What am I missing here? Also ran <code>setInternet2(TRUE)</code> first, but got identical results. </p>