signal.c: send signal unless handled

* signal.c (rb_f_kill): should not ignore signal unless the
  default handler is registered.  [ruby-dev:48592] [Bug #9820]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47818 b2dd03c8-39d4-4d8f-98ff-823fe69b080e