Integer list option custom fields do not render properly in reports

To reproduce :

• create a custom fields on contact of type Integer / Select
• go to a contact report and try to filter by this custom field
  You get a numeric filter instead of a select filter.

The reason why we use a Integer instead of a String is for database space optimization but we don't want the Integer to be shown anywhere. It has no meaning to the end user.

In the advanced search, for the same custom field, the filter is correct and use a html select.