Fixed description for zsuper in define_method method.
Fixed Proc specs for MRI 2.1.0.
Fixed String specs for MRI 2.1.0.
Removed invalid spec.

Given the insanity of _id2ref, there is no guaranteed valid way to generate
an invalid reference to trigger this behavior so it is left undefined.
Removed invalid spec.

Since immediates are all frozen by default in MRI 2.1, this spec causes
a RuntimeError trying to modify a frozen value.