title: UrlFetchApp.fetch() doesn't seem to change user agent
body: <p>Trying to grab data from a website using Google Apps Script to put it directly into a spreadsheet. The fetch does not seem to be working, where the Python requests equivalent works just fine.</p>
  
  <p>Python code:</p>
  
  <pre class="lang-py prettyprint-override"><code>page = requests.get("someurl?as_data_structure", headers={'user-agent':'testagent'})
  </code></pre>
  
  <p>GAS code:</p>
  
  <pre><code>var page = UrlFetchApp.fetch("someurl?as_data_structure", headers={'user-agent':'testagent'});
  </code></pre>
  
  <p>The only required header is the user-agent, and the error I am getting from the GAS code is what I would usually get from the Python code if I hadn't included the header. I am new to js but as far as I know this is the proper way to do it..?</p>
  
  <p>EDIT:
  Now got the headers in the right place but the issue persists, exactly the same error as before.</p>
  
  <pre><code>var options = {"headers": {"User-Agent": "testagent"}};
  var page = UrlFetchApp.fetch("someurl?as_data_structure", options);
  </code></pre>