libs/web: fix luci.http.write_json() to properly encode control chars in strings

libs/web/luasrc/http.lua

@@ -4,9 +4,6 @@ LuCI - HTTP-Interaction
 Description:
 HTTP-Header manipulator and form variable preprocessor
 
-FileId:
-$Id$
-
 License:
 Copyright 2008 Steven Barth <steven@midlink.org>
 
@@ -340,6 +337,8 @@ function write_json(x)
 			write(tostring(x))
 		end
 	else
-		write("%q" % tostring(x))
+		write('"%s"' % tostring(x):gsub('["%z\1-\31]', function(c)
+			return '\\u%04x' % c:byte(1)
+		end))
 	end
 end