merge revision(s) r46896,r46897,r46898: [Backport #10078]

	* string.c (rb_str_count): fix wrong single-byte optimization.
	  7bit ascii can be a trailing byte in Shift_JIS.
	  [ruby-dev:48442] [Bug #10078]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@47255 b2dd03c8-39d4-4d8f-98ff-823fe69b080e