Correct use of EdmTypeCannotBeNull message (fixes #1558)
- https://aspnetwebstack.codeplex.com/workitem/1558
- "[OData] Incorrect use of EdmTypeCannotBeNull message"
- Make three uses of this resource string consistent
- PLOC issue primarily in EntityInstanceContext.GetPropertyValue() because message
mentioned "an System.Web.Http.OData.IEdmObject"
- Problem more than a PLOC issue since ODataSerializerContext.GetEdmType() would
throw NRE not intended Exception and ODataMediaTypeFormatter.GetSerializer()
would reference incorrect Type in Exception message
- Also add tests covering these sad paths (2 were missed)

(cherry picked from commit 3c305a0baad8af864c04b7a79a8652ca49760b50)