fix the filter_groups keys non-string BUG

I met the same problem as issue [#245](https://github.com/mrjoes/flask-admin/issues/245), and I notice the fix. But the new 'filter_groups' stuff in commit [d851c420019df6212d576f31301519cfb12104dc](https://github.com/mrjoes/flask-admin/commit/d851c420019df6212d576f31301519cfb12104dc) seems did nothing to it's non-string keys for 'tojson' function in the [template/admin/model/list.html ](https://github.com/mrjoes/flask-admin/blob/master/flask_admin/templates/bootstrap3/admin/model/list.html).

this is my PR based on the previous fix, hope you can find a better way to solve this, Thanks.