This change introduces a new property to ModelMetadata called
IsBindingRequired, which specifies whether or not a model value must be
present on the wire during model binding.
[DataMember(IsRequired = true)] is currently the only thing that will set
this property.
Updated tests and documentation for clarity on the difference in meaning
between MM.IsRequired and MM.IsBindingRequired. Moved setting for
IsRequired to ValidationMetadata which is a better fit.
Also added functional tests for [BindingBehavior] and [DataMember] in
model binding because they were totally missing.
