add IE9 support via trivial fix.

Author: sorvell

src/HTMLImports.js

@@ -423,7 +423,7 @@ xhr = xhr || {
     request.addEventListener('readystatechange', function(e) {
       if (request.readyState === 4) {
         next.call(nextContext, !xhr.ok(request) && request,
-          request.response, url);
+            request.response || request.responseText, url);
       }
     });
     request.send();