The move to the requests library (dbb242b) broke the progress bar during
downloading an image with --progress enabled, due to requests returning
a generator, which has no __len__ function. This patch adds an iterable
wrapper which provides the length, sourced from Content-Length headers.
Closes-Bug: #1384664
Change-Id: I48598a824396bc6e7cba69eb3ce32e7c8f30a18a