Commit 3bd239e9 makes it so we set the
object's last-modified timestamp to the current timestamp if there's
no Last-Modified header.  This makes it so we can do conditional gets
even when there's no Last-Modified or ETag header from the backend.
However, the time entered was a float, so we effectively ended up with
an off-by-up-to-one-minus-epsilon error.  This lead to requests ending
up in a 304/200 response code round-dance.

We now round the timestamp down to the nearest whole second instead,
which should fix this bug.  Thanks to lrowe for a helpful bug report.

Fixes: #907