Since the Related-Change, swob's date_header_format function will
accept Timestamp instances.
Drive-by: call out the use of Timestamp.ceil() in the
date_header_format docstring.
Change-Id: If45a8172d225eea70160f5034e337feb027bda2b
Related-Change: Ie7090e8dbc90e814f74fbcbd4fba2ca40ee1fcc7
Signed-off-by: Alistair Coles <alistairncoles@gmail.com>