Handle setting None consistently in extensions.

[lossyrob]

In extension classes there is an apply method for setting all the properties of the extension at once. These will take optional parameters for optional properties, normally defaulting to None.

We should be consistent across all the extensions about whether or not to use the None value as an indication that the optional property should be unset.

We should also ensure that any optional property setters will take None as a value and unset the property accordingly.

[lossyrob]

Fixed by #272