* hash.c (hash_aset_str): Use rb_fstring() to de-duplicate hash string
  keys. Patch by Eric Wong. [Bug #8998] [ruby-core:57727]
* test/ruby/test_hash.rb (class TestHash): test for above.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43870 b2dd03c8-39d4-4d8f-98ff-823fe69b080e