(#20681) delete_at function unit test against issue

The issue #20681 describe the error of delete() function
removing the elements from the origin array/hash/string.

This issue affected the other delete functions.

The delete_at function is not afected by this bug, but
it did not had the unit test to check against it.

I had added the unit test so we could prevent regressions
on the future and also have better test coverage.
Merge pull request #183 from lmello/fix_bug_20681_delete_at

delete_at added spec to check against bug #20681