Add test case for BigDecimalRowKey with a variety of +/- exponents.

Appears to show that serialized bytes of BigDecimal do *not* sort in natural order.  Unless I am missing something?

Rest results:

Expected :[-0.5555555555555555555555555555555555, -0.5555555555555555, -0.555555555555555,
Actual   :[-0.55, -0.5555, -0.555555, -0.55555555,