augment check for string data for object as well

GCM allows for more than just `string` key/value pairs.

From the docs (http://developer.android.com/google/gcm/gcm.html):

The values could be any JSON object, but we recommend using strings, since the values will be converted to strings in the GCM server anyway.

This pull request augments the `string` check to also allow an object through the filter.

Thanks!