Fixing reject not being called on xhr network errors.

Fixes #204

lib/adapters/xhr.js

@@ -63,6 +63,16 @@ module.exports = function xhrAdapter(resolve, reject, config) {
     request = null;
   };
 
+  // Handle low level network errors
+  request.onerror = function handleError() {
+    // Real errors are hidden from us by the browser
+    // onerror should only fire if it's a network error
+    reject(new Error('Network Error'));
+
+    // Clean up request
+    request = null;
+  };
+
   // Add xsrf header
   // This is only done if running in a standard browser environment.
   // Specifically not if we're in a web worker, or react-native.