merge revision(s) 42178: [Backport #8687]

	* rational.c (f_round_common): Rational is expected to be returned by
	  Rational#*, but mathn.rb breaks that assumption.  [ruby-core:56177]
	  [Bug #8687]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@42217 b2dd03c8-39d4-4d8f-98ff-823fe69b080e