Fix sorting of menu items whose labels are procs

Commit 452956eccf77ea368b05902a3d0d8020152a7aaa introduced a bug in `MenuItem#<=>`: even if the label was a proc, it would not compare `label.call <=> other.label.call`, but instead `label.to_s <=> other.label.to_s`, and `Proc#to_s` returns some irrelevant metadata about the Proc object.

This bug would show when the menu items got randomly rearranged on every refresh of the admin area.