dashenc: fix ISO8601 UTC parsing

Appends Z to timestamp to force ISO8601 datetime parsing as UTC.
Without Z, some browsers (Chrome) interpret the timestamp as
localtime and others (Firefox) interpret it as UTC.
Signed-off-by: NAnton Schubert <ischluff@mailbox.org>
Signed-off-by: NMartin Storsjö <martin@martin.st>

libavformat/dashenc.c

@@ -429,7 +429,7 @@ static void format_date_now(char *buf, int size)
     struct tm *ptm, tmbuf;
     ptm = gmtime_r(&t, &tmbuf);
     if (ptm) {
-        if (!strftime(buf, size, "%Y-%m-%dT%H:%M:%S", ptm))
+        if (!strftime(buf, size, "%Y-%m-%dT%H:%M:%SZ", ptm))
             buf[0] = '\0';
     }
 }