Add a good test for integers with overpadded zero values.

[tgregg]

See:

• Clarify spec to allow overpadded zero integers. ion-docs#93
• Allow non-compact binary encodings of int zero ion-docs#96

0x20 is the most common binary form of int 0, and is the only form that standard Ion writers should write. However, overpadded encodings of 0 with the positive int type code 2 are permitted, e.g. 0x21 0x00. Add a test for such overpadded encodings.