Coded alternative for array_diff_assoc as this method does not support m...

Added an alternative method for array_diff_assoc as this method does not support multidimensional arrays. The alternative method serializes the embedded arrays and then compares using the built in php methods. The result is then unserialized before being returned.

This will get rid of the annoying warning
"Notice: Array to string conversion in islandora_solr_admin_settings()"

Also it should make the code work as intended as the current solution will always return an empty array claiming no difference even if this is not the case.