Add specs for BigDecimal#round with NaN and Infinity

Hello,

This pull request simply adds specs that were added to address a regression on JRuby. I'm not familiar with specs-writing so let me know if I should rephrase anything!

Refs jruby/jruby#1808.

Have a nice day.
Ok last nit-picky thing: do we have to use a Regexp here or can we use just a String? If the latter then it would remove the need for escaping the parenthesis.