httplib's putheader method will cast whatever you give it to a string.
where we allow the default dict.get default of None to be passed to
putheader unmodified ssync_receiver is surpised that the non-empty
string isn't able to be converted to an integer.
We can avoid surprising the ssync_receiver in this way by sending the
empty string as a better default.
Change-Id: Ie9df9927ff4d3dd3f334647f883b2937d0d81030
d124ce5792