Fix for #13175.
This change allows user-input timestamps (e.g. to `docker events
--since/--until` or `docker logs --since` to be parsed using
standard RFC3339Nano layout in Go instead of the layout that parses
all timestamps into fixed-length strings (currently buggy).
User inputs need not to be complying to the internal format
(`RFC3339NanoFixed`) anyway.
Added test case for `events --since/--until` with all possible
timestamp input formats.
Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>
