Fix invalid SQL exception when using field_search to filter against a relation column that is not in desired set of export columns.

Make sure active_scaffold's find_page is dealing with the same list of
columns. Prevents an invalid SQL query when exporting after filtering with
field_search against a relation column, and that relation column is not
included in the set of export columns.

https://groups.google.com/forum/#!topic/activescaffold/S_5fZ3DTjQY