* Add test for device_merge with mismatched iterators

Add a test case (failing) for using device_merge with different types for
`keys_input1` and `keys_input2`. This is supported and should work.

* Fix device_merge with mismatched input iterators

Co-authored-by: Gergely Meszaros <gergely@streamhpc.com>

Co-authored-by: Vincent van Heertum <93536747+vince-streamhpc@users.noreply.github.com>
Co-authored-by: Gergely Meszaros <gergely@streamhpc.com>