Fix promise being rejected on IE 9 when server response is invalid

Reject all HTTP codes that are not in the 100..599 range.

When an empty reply is received from the server, `xhr.onerror` callback
should get triggered. IE 9, however, triggers the `onload` callback with
status code 12152 that stands for:

    ERROR_HTTP_INVALID_SERVER_RESPONSE
    The server response could not be parsed.

http://support.microsoft.com/kb/193625

fetch.js

@@ -178,8 +178,13 @@
       var xhr = new XMLHttpRequest()
 
       xhr.onload = function() {
+        var status = xhr.status
+        if (status < 100 || status > 599) {
+          reject()
+          return
+        }
         var options = {
-          status: xhr.status,
+          status: status,
           statusText: xhr.statusText,
           headers: headers(xhr)
         }

test/test.js

@@ -47,7 +47,10 @@ asyncTest('resolves promise on 500 error', 2, function() {
 })
 
 asyncTest('rejects promise for network error', 1, function() {
-  fetch('/error').catch(function() {
+  fetch('/error').then(function(response) {
+    ok(false, 'HTTP status ' + response.status + ' was treated as success')
+    start()
+  }).catch(function() {
     ok(true)
     start()
   })