Use msgstr[0] when stringifying an untranslated plural item.

lib/po.js

@@ -321,8 +321,11 @@ PO.Item.prototype.toString = function () {
                     lines = lines.concat(mkObsolete + _process(keyword, t, i));
                 });
             } else {
+                var index = (self.msgid_plural && Array.isArray(text)) ?
+                    0 :
+                    undefined;
                 text = Array.isArray(text) ? text.join() : text;
-                var processed = _process(keyword, text);
+                var processed = _process(keyword, text, index);
                 //handle \n in single-line texts (can not be handled in _escape)
                 for (var i = 1; i < processed.length - 1; i++) {
                     processed[i] = processed[i].slice(0, -1) + '\\n"';