Custom fields do not respect permissions when added to reports

When a report class defines $_customGroupExtends and includes custom fields in Display Columns / Filters - users' permissions to access that custom data set should be checked and the fields should NOT be included if the user doesn't have access.

To recreate the bug:

• Modify sample custom data field - Donor Info->How Long Have You Been a Donor so that the field Is Searchable.
• Create a Drupal role that has access CiviReport, access CiviContribute, view all contacts. BUT role does NOT have 'access all custom data'
• Log in as that role and view or edit a contribution record. The Donor Info custom data set is NOT shown.
• Now go to Donor Report (Detail)
• The custom data set IS included in Display Columns and Filters AND values can be viewed on the report.