$entity should require cast segment in order to apply $select/$expand

Given an opaque id for an entity, the client can get the instance using the new /$entity resource and providing the id through the $id query option, such as:

http://host.service/$entity?$id=urn:mysvc5304-98001-7832-4425

This can be cast to a particular type:

http://host.service/$entity/myservice.Customer?$id=urn:mysvc5304-98001-7832-4425

And any of the query options supported for a single resource can be applied:

http://host.service/$entity/myservice.Customer?$id=urn:mysvc5304-98001-7832-4425?$select=LastName&$expand=Orders

However the text current does no require the cast in order to apply $select and $expand. This is a bug, because without the cast a parser cannot determine whether the $select and $expand are valid without actually retrieving the object and discovering its type.

Everywhere else we consistently require casting to the correct type in order to access members of that type.

Also, I think there is a bug in the ABNF as it doesn't appear to allow for the cast segment:

'$entity' "?" entityOptions