This change adds support for our three-valued logic to the default value
handling part of the MutableObjectModelBinder.
The issue is that we want to look up a default value when a 'greedy' model
binder returns true but doesn't find a value.
We also don't want to call the property setter unless there is:
1). A value from model binding OR
2). A default value
See #1695 for a detailed explanation. This change builds support into the
system for the case that a model binder returns true without setting a
value for the Model.
In this case, validation will be skipped if it's a top-level object.
Note that explicitly setting null will still run validation.
The issue here is that a model state error is added with the model name
'doubled'. This is on a fairly obscure code path and the code dates back
to the original WSR git checkin of webapi. There are no wsr tests that
verify this behavior.
The cause here is that our 'greedy' model binders (like
FromHeaderModelBinder) return 'true' whether or not they successfully
found a model, because they don't want other model binders to run.
This also has an effect on the validation system. That means that
validators will run and attempt to validate the model (which may be null).
That's that rare case where we get to this code path.
Ryan Nowak