Provide tests that '8' and '9' are seen as invalid octal characters.

The ruby octal exercise needs tests to verify that a submitted octal string, like "89" is converted to 0 by #to_decimal.

A mixture of valid and invalid characters is not tested. In fact "6789" would be converted to 3520 with the current example code. I have not provided a test to assert that "6789" should convert to 0. It is not clear from the provided tests and example code that conversion to 0 is considered correct behavior by the author. 

I can provide tests and alternate example code to convert any string containing invalid octal characters to 0, if requested.