* ext/bigdecimal/bigdecimal.c (BigDecimal_power): Round the result value.
  [Bug #8818] [ruby-core:56802]

* test/bigdecimal/test_bigdecimal.rb: Add a test for the above fix.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43796 b2dd03c8-39d4-4d8f-98ff-823fe69b080e