Why doesn't mocha stub Tempfile#unlink?

tempfile-unlink-mocha.rb

require 'mocha/api'
t = Tempfile.new %w[ temp .file ]

## show that another method on a tempfile can successfully be stubbed
t.stubs(:closed?).returns :something_it_would_not_normally_return
t.closed? #=> :something_it_would_not_normally_return

## show that an imaginary method on a tempfile can successfully be stubbed
t.stubs(:wiggly?).returns :yes_of_course
t.wiggly? #=> :yes_of_course

## show that Tempfile#unlink specifically cannot be stubbed
t.path #=> not nil, as is reasonable, it hasn't been unlinked
t.stubs(:unlink).returns :another_return_value
t.unlink #=> t
t.path #=> nil, because the actual unlink was called

## show that unlink can be stubbed on other objects
o = Object.new
o.stubs(:unlink).returns :i_am_not_linked
o.unlink #=> :i_am_not_linked