delete_at added spec to check against bug #20681

   (#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.