Add specs for Module#remove_method
Fix Module#remove_method
Add specs for a C-API rb_undef_method
Clarify to check frozen on Module#undef_method!

Module#undef_method! is raising exceptions from
@method_table frozen status.
This commit clarifies "the check is need here".
Add specs for Module#undef_method
Fix Module#undef_method

https://github.com/rubinius/rubinius/pull/2467#discussion_r5294311