GeojsonMixin: fix case-insensitive comparison

[pjonsson]

What this PR does

Add a return before the boolean expression to make the behavior different for case-insensitive mode.

Test me

Not sure where this is used.

Checklist

• [ ] There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• [ ] I've updated relevant documentation in doc/.
• [ ] I've updated CHANGES.md with what I changed.
• [ ] I've provided instructions in the PR description on how to test this PR.