after upgrade-charm, new config settings do not all have default values

Seems on upgrading a charm the defaults from config.yaml are not populated into the configs of a service. This can cause major problems for config-changed hooks that assume those values will be set to something that makes sense, especially in the case of 'int' config values which should always, by definition, have an integer value.

Test case:

deploy an empty charm
add a config.yaml with one setting

options:
  foo:
    type: int
    default: 1
    description: fooberries
  bar:
    type: string
    default: "baz"
    description: "bar-tastic"

upgrade-charm to pick this config change up, the start debug-hooks and run

juju set myservice bar="something"

In debug-hooks

# config-get --format=json
{"bar": "something"}
#

No defaults!

juju:
  Installed: 0.5+bzr427-1juju2~oneiric1
  Candidate: 0.5+bzr427-1juju2~oneiric1
  Version table:
 *** 0.5+bzr427-1juju2~oneiric1 0
        500 http://ppa.launchpad.net/juju/pkgs/ubuntu/ oneiric/main i386 Packages
        100 /var/lib/dpkg/status
     0.5+bzr398-0ubuntu1 0
        500 http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ oneiric/universe i386 Packages