There was a bug where the `www-authenticate` header in the HTTP response
from a registry would not be parsed accurately.
Specifically, if the header value had more than one `<space>` character,
the entire header would be ignored. This PR fixes this bug and adds unit
test to detect this in the future.
Fixes https://github.com/apple/container/issues/240
And most likely fixes https://github.com/apple/container/issues/237
Signed-off-by: Aditya Ramani <a_ramani@apple.com>