vm.c: return the result hash

* vm.c (core_hash_merge_kwd): should return the result hash, which
  may be converted from and differ from the given argument.
  [ruby-core:62921] [Bug #9898]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46342 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-06-04

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46343 b2dd03c8-39d4-4d8f-98ff-823fe69b080e