Support writing enum-valued Annotations

[mikepizzo]

ODataLib doesn't support writing annotations whose value is an enum.

Assemblies affected

OData .Net lib 6.x, OData.NET lib 7.x.

Reproduce steps

ODataProperty property = new ODataProperty { Name="Name", Value="Dan"};
property.InstanceAnnotations.Add(new ODataInstanceAnnotation(CoreVocabularyModel.PermissionsTerm.FullName(), new ODataEnumValue("Read", "Org.OData.Core.V1.Permission")));

Expected result

Annotation should be written:

{
    "Name":"Dan",
    "Name@Org.OData.Core.V1.Permissions": "Read"
}

Actual result

Exception is thrown.

[xuzhg]

Fix at #815. Thanks!

[xuzhg]

Close it.