replacing a hash with itself returns an empty hash in ruby 1.9 compatibility mode

jruby (ruby 1.9 mode):
> h = {:one => 1}
=> { :one => 1 }
> h.replace h
=> {}

mri (1.8.7 through 2.0.0-rc2) and jruby (ruby 1.8 mode):
> h = {:one => 1}
=> { :one => 1 }
> h.replace h
=> { :one => 1 }