#9316 BigDecimal division in Ruby 2.1

This is a *failing* test that illustrates the problem.

./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb test/bigdecimal/test_bigmath.rb
...
[4/6] TestBigMath#test_division = 0.00 s     
  1) Failure:
TestBigMath#test_division [test/bigdecimal/test_bigmath.rb:18]:
Expected |1486.868686869 - 1487.0| (0.13131313100006992) to be <= 0.001.