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.
Yep fair point. This is the case on the JRuby pull request ; I don't know why I forgot it there. Updated and rebased ; thanks! :-)