Downloading image with --progress fails with "RequestIdProxy object is
not an iterator". This is because to display download progress
VerboseFileWrapper in progressbar requires object of IterableWithLength,
but after support of returning request-id [1] to caller it returns
RequestIdProxy object which is wrapped around IterableWithLength
and response.
To resolve this issue overridden next and __next__ methods in
RequestIdProxy so that it can act as iterator for python 2.x
and 3.x as well.
[1] 610177a779
Closes-Bug: #1670464
Change-Id: I188e67c2487b7e4178ea246f02154bbcbc35a2b1
60c06d526c