The course shortname in a RSS header ignores multilang filter.

REPRODUCTION:
(1) Enable multilang-filter for content and headings and enable RSS
(2) Create a course using multilang-tags for its shortname (<- the shortname field is just limited up to 100 chars, so the "long" multilang-tag could lead to a problem. But how to solve that is already reported in another issue!)
(3) In this course, create a database activity, choose a preset and add an entry
(4) Open RSS feed for this activity
(5) Look at the heading, the course shortname is displayed in all languages

FIX:
Add a "format_string()" to mod/data/rsslib.php line 83:

$header = rss_standard_header(format_string($course->shortname).': '.format_string($data->name,true),

ADDITIONAL INFORMATION:
The RSS feeds for glossary and forum are displayed correctly. There, the code fix above is already present.