Labels to keep field in plan configuration case sensitiveness problem

Steps to reproduce:

1. Disable Global Expiry (Admin >> Plans >> Build Expiry, mark 'Disable Global Build Expiry', and click 'Save')
2. Run a plan multiple times
3. Open some build results (Dashboard >> Plan name >> Build result number) and add some labels in lowercase, like: persist keep donotdelete
4. Edit the plan, and go to Miscellaneous tab
5. Mark 'Don't expire builds with certain labels', enter PERSIST KEEP DONOTDELETE in the 'Labels to keep' field, and click 'Save'
6. Go to Admin >> Plans >> Build Expiry, and force Build Expiry to run using the 'Run' button.
7. The build results you entered labels in step 3 above were deleted.

If you had entered lowercased labels in step 5 above, the build results you entered labels in step 3 above wouldn't have been deleted.

This proves that "Labels to keep" field is case sensitive.

The workaround is to ensure the labels are always entered in lowercase.