This patch fixes downloading files to stdout and modifies
_SwiftReader to operate as an iterator that performs file
checks at the end of iteration as well as a context manager.
File verification checks have been removed from __exit__
and added to __iter__.
Change-Id: I3250bdeeef8484a9122c4b5b854756a7c8f8731e
Closes-Bug: 1395922
Closes-Bug: 1387376