[Codeplex #1653]: For bound actions, the combination of action name and
the binding parameter type MUST be unique within a namespace.

In this fix, a validation function added in GetEdmModel() to validate the
action overload:

1. For bound action overload, each overload action must have differnt
bingding type.
2. For unbound action, each unbound action must have different action
name.

Thanks.