merge revision(s) 6375c68f,c05eaa93: [Backport #15839]
	parse.y: function parser_mixed_error & parser_mixed_escape
	
	Fix mixed encoding in heredoc
	
	Heredocs are parsed line-by-line, so we need to keep track of the
	temporary encoding of the string.  Previously, a heredoc would
	only detect mixed encoding errors if they were on the same line,
	this changes things so they will be caught on different lines.
	
	Fixes [Bug #15839]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@67763 b2dd03c8-39d4-4d8f-98ff-823fe69b080e