OData ATOM format should require <category> element

The <category> element is used in atom to identify the type of the individual entity. Many clients rely on this element in order to understand the type of the individual entry.

The atom specification currently says this element is optional if the type of the entity can be determined from the feed (in particular, if there is no inheritance hierarchy):

"An OData entry MAY contain a single atom:category element with a scheme attribute equal to "http://schemas.microsoft.com/ado/2007/08/dataservices/scheme" to identify the entity type of the entry. This element MUST be present if the entry represents an entity whose type is part of a type hierarchy."

This optionality adds complexity to the client logic, as clients don't have a single place to look to determine the type; they have to understand more context in order to determine the type in the absence of the element.

The savings of omitting this element is negligible for the ATOM format.