Fix expectation for the result of "\0".inspect in MRI 2.0.0
At r40488, MRI 2.0.0 head behaves like 1.9.3 and before about String#inspect
with NUL character.
Merge pull request #5 from nagachika/string_inspect_for_mri_2_0_0_head

Fix expectation for the result of "\0".inspect in MRI 2.0.0