Commented Unassigned: Enum support in WebApi [1584]

WebApi treats Enum type as String and processes Enum type specially.
Since OData V4 supports Enum type, we want to support Enum type in WebApi after OData migrates to V4.
ODataModelBuilder, ODataConventionModelBuilder, serialization, deserialization and query should be modified to support this feature.
Comments: Checked in: commit ec4708259b03198f9082997e8e86d1a6af96df17 webapi.odata: v4 work for enum in query $filter 1. Add Enum support in filter binder 2. Add support for Enum "has" operator 3. Update & enable unit tests