merge revision(s) 42820: [Backport #8856]

	* string.c (str_gsub): use BEG(0) for whole matched position not
	  return value from rb_reg_search(), for \K matching.
	  [ruby-dev:47694] [Bug #8856]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@42915 b2dd03c8-39d4-4d8f-98ff-823fe69b080e