Fixing issue with responseURL property

`responseURL` is not supported in IE

lib/adapters/xhr.js

@@ -57,7 +57,7 @@ module.exports = function xhrAdapter(config) {
       // handled by onerror instead
       // With one exception: request that using file: protocol, most browsers
       // will return status as 0 even though it's a successful request
-      if (request.status === 0 && request.responseURL.indexOf('file:') !== 0) {
+      if (request.status === 0 && !(request.responseURL && request.responseURL.indexOf('file:') === 0)) {
         return;
       }