shortcode fails using Ajax + admin-ajax.php

This fixes the issue where the ninja_forms_display_form shortcode would not work if the form is called via Ajax, using the recommended handler: admin-ajax.php.
See for more info : http://wordpress.stackexchange.com/questions/170808/the-content-shows-shortcode-instead-of-parsing-it/170820?noredirect=1#comment248485_170820

Please credit Milo (http://wordpress.stackexchange.com/users/4771/milo)
Merge pull request #396 from pixeline/patch-1

shortcode fails using Ajax + admin-ajax.php