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merge revision(s) 49090: [Backport #10686]

    * symbol.c (rb_gc_free_dsymbol): delete from global fstr hash

    * test/ruby/test_symbol.rb (test_symbol_fstr_leak): test for bug
      [ruby-core:67268] [Bug #10686]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@49274 b2dd03c8-39d4-4d8f-98ff-823fe69b080e