Fix sorting of menu items whose labels are procs

Commit 452956eccf77ea368b05902a3d0d8020152a7aaa introduced
a bug in `MenuItem#<=>`: 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.