title: jQuery AJAX GET/POST request returns 404 in error handler, but valid response is sent from server
body: <p>The Problem: I send GET or POST request with jQuery.ajax and get an error handler triggered with 404. The thing is, I have used a sniffer to check an answer from server and it is a valid 200 response with a correct JSON (returned by Python json.dumps).
  And something weird: after this call ends up with 404 a browser reloads the page. </p>
  
  <p>Everything lives in the same domain (and subdomain too).</p>
  
  <p>jQuery call:</p>
  
  <pre><code>$.ajax({type: "POST", url: "/m/auth/login", data: {x: "y"},
  success: function(result) {}, error: function(xhr) {}, dataType: "json"});
  </code></pre>
  
  <p>Response as seen by sniffer with payload decompressed:</p>
  
  <pre><code>HTTP/1.1 200 OK\r\n
  Content-Type: text/html; charset=utf-8\r\n
  Cache-Control: no-cache, must-revalidate\r\n
  Content-Encoding: gzip\r\n
  X-AppEngine-Estimated-CPM-US-Dollars: $0.000018\r\n
  X-AppEngine-Resource-Usage: ms=71 cpu_ms=0\r\n
  Vary: Accept-Encoding\r\n
  Date: Tue, 24 Dec 2013 21:04:36 GMT\r\n
  Pragma: no-cache\r\n
  Expires: Fri, 01 Jan 1990 00:00:00 GMT\r\n
  Server: Google Frontend\r\n
  Content-Length: 80\r\n
  Alternate-Protocol: 80:quic,80:quic\r\n
  \r\n
  {"reason": {"password": "missing", "email": "missing"}, "error": "argument"}
  </code></pre>