1056 Make refresh work when foreign key is set to null, and with load_json

[dantownsend]

Related to #1056

There are a couple of other edge cases I discovered.

Null objects

>>> band = await Band.objects(Band.manager).where(Band.name == "Pythonistas").first()
>>> band.manager.name
"Guido"

# If the band.manager foreign key gets set to null, when we refresh the object, it should now be null
>>> await band.refresh()
>>> band.manager
None

load_json

When refreshing, it doesn't load any JSON:

>>> studio = await RecordingStudio.objects().output(load_json=True).first()
>>> studio.facilities
{'restaurant': True, 'mixing_desk': True}

>>> await studio.refresh()
>>> studio.facilities
 '{"restaurant":true,"mixing_desk":true}'

Now you can do this:

>>> await studio.refresh(load_json=True)
>>> studio.facilities
{"restaurant":true,"mixing_desk":true}