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.
Harsh Gupta
Ryan Nowak