Convert UTF8 encoded passwords to ISO-8859-1 for |R = 6| encryption (issue 6010)

For passwords where the encoding already is correct, the conversion is a no-op.
Also, since `encodeURIComponent` might throw, we need to make sure that we handle that case too.

Fixes 6010.

src/shared/util.js

@@ -946,6 +946,10 @@ function stringToUTF8String(str) {
   return decodeURIComponent(escape(str));
 }
 
+function utf8StringToString(str) {
+  return unescape(encodeURIComponent(str));
+}
+
 function isEmptyObj(obj) {
   for (var key in obj) {
     return false;