restore previous behaviour with \n in strings

An incompatible change (that actually breaks po parsing after writing) had
been introduced with commit e164fcfe9d. If
_process returned an array (which is the case for strings containing \n
character), array.toString will return a comma separated list, which is not
valid po syntax. Added a test to restore the behaviour from before the
e164fcfe9d.

lib/po.js

@@ -261,7 +261,8 @@ PO.Item.prototype.toString = function () {
                 });
             } else {
                 text = isArray(text) ? text.join() : text;
-                lines = lines.concat(mkObsolete + _process(keyword, text));
+                var processed = _process(keyword, text);
+                lines = lines.concat(mkObsolete + processed.join('\n' + mkObsolete));
             }
         }
     });