Fix BigDecimal tests on master

@atambo looks like not all of 284eddc9d440f2968d9bd4b6989c24ab4c4e8555 made it over to master intact when merged, which left BigDecimal in a bad state.  

In particular, tests `TestBigDecimal#test_math_extension` and `TestBigDecimal#test_big_decimal_mode` were broken.  This fixes them up.

Note: those aren't the only tests having a rough time on master right now, so this won't quite get Travis green... will try to have a look at the other problems soon too.