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.
Is there a specific reason for only testing a part of the error message like this? I think we should either test the whole message or not test the message at all.