get-bugzilla-attachments: RSS query doesn't work with '+' character

eg. image/xml+svg

Change-Id: I1b6b6f71d02fa415c3470faadbe8d316aaf0f677
Reviewed-on: https://gerrit.libreoffice.org/19761Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

bin/get-bugzilla-attachments-by-mimetype

@@ -247,7 +247,7 @@ def get_through_rss_query(queryurl, mimetype, prefix, suffix):
                 print(entry['id'] + " failed: " + str(sys.exc_info()[0]))
                 pass
 
-    query = create_query(escape(mimetype))
+    query = create_query(escape(mimetype.replace("+","%2B")))
     query['ctype'] = 'rss'
 
     files = get_downloaded_files(prefix, suffix)