Fix XmppDateTime parsing of fractional seconds

XEP-82 states that the fractional second abbendum MAY contain any number
of digits. Previously XmppDateTime was able to handle 0, 3, or more then
3 digits, but not 1 or 2.

Now we fill up the fractional seconds with zeros if the date string has
1 or 2 fractional digit seconds.

Thanks to Holger Weiß for pointing this out.