change the last-modified header value with valid one

the Last-Modified header in Response didn't have a suitable
value - an integer part of object's timestamp.
This leads that the the if-[un]modified-since header with the
value from last-modified is always earlier than timestamp
and results the content is always newer than value of these
conditional headers.
Patched code returns math.ceil() of object's timestamp
in Last-Modified header so the later conditional header works
correctly

Closes-Bug: #1248818
Change-Id: I1ece7d008551bf989da74d23f0ed6307c45c5436
Merge "change the last-modified header value with valid one"