looks like jruby and rubinius would copy strings for hash keys

so we can't really use strings to test this... unless we're only
aiming for mri. not sure if copying string keys would be a good
idea.... it kinds of make sense, but not really as what i would expect.

    str = 'str'
    h = {}
    h.compare_by_identity
    h[str] = h[str] = true
    h.size # 2 in jruby and rubinius, 1 in mri

in jruby and rubinius, we would end up with 3 strings...