Edited Issue: Support Enum in QueryableAttribute [429]

Since we already support enum type in model builder (converting it to string), it will be useful to support enum property in query composition as a string property.

Consider following user scenarios:

/Products?$filter=Currency eq 'USD'
For flags: /Products?$filter=substringof('Favorite', Tags)

Currently, QueryableAttribute will report: missing non-standard type support for <enum type> exception.