merge revision(s) 40920: [Backport #8450]

	* string.c (rb_str_quote_unprintable): check if argument is a string.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@41067 b2dd03c8-39d4-4d8f-98ff-823fe69b080e