* 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/trunk@39967 b2dd03c8-39d4-4d8f-98ff-823fe69b080e