Added specs for rb_io_taint_check().

Added for #3057.
Implement rb_io_taint_check().

Unlike as the name might suggest this function checks if an object is frozen or
not. The tainted status is completely ignored. See
http://rxr.whitequark.org/mri/source/io.c?v=2.1.2#600 for the MRI reference
code.

This fixes #3057.