If a user specifies `mydomain.com:443` in the `Host` configuration, the
PATCH request for the layer upload will fail because the challenge does not
appear to be in the map. To fix this, we normalize the map keys to always
use the Host:Port combination.
Closes https://github.com/docker/docker/issues/18469
Signed-off-by: Stan Hu <stanhu@gmail.com>