merge revision(s) 39967: [Backport #8193]

	* time.c (num_exact): use to_r method only if to_int method is
	  available.
	  [ruby-core:53764] [Bug #8173] reported by Hiro Asari.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@40298 b2dd03c8-39d4-4d8f-98ff-823fe69b080e