merge revision(s) r47190: [Backport #10139]

	* string.c (setup_fake_str): fake strings should not set class by
	  RBASIC_SET_CLASS() because it insert write barriers to fake
	  (non-RVALUE) structure.
	  It can cause unexpected behaviour.
	  Ruby 2.1 also have a same problem (setup_fake_str() in parse.y).

	* symbol.c (setup_fake_str): ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@47480 b2dd03c8-39d4-4d8f-98ff-823fe69b080e