add omit-values supports

OData specs has the following at: http://docs.oasis-open.org/odata/odata/v4.01/cs01/part1-protocol/odata-v4.01-cs01-part1-protocol.html#_Toc505771137

 8.2.8.6 [Preference omit-values](http://docs.oasis-open.org/odata/odata/v4.01/cs01/part1-protocol/odata-v4.01-cs01-part1-protocol.html#sec_Preferenceomitvalues)
The omit-values preference specifies values that MAY be omitted from a response payload. Valid values are nulls or defaults.

If nulls is specified, then the service MAY omit properties containing null values from the response, in which case it MUST specify the Preference-Applied response header with omit-values=nulls.

If defaults is specified, then the service MAY omit properties containing default values from the response, including nulls for properties that have no other defined default value. Nulls MUST be included for properties that have a non-null default value defined. If the service omits default values it MUST specify the Preference-Applied response header with omit-values=defaults.

Properties with null or default values MUST be included in delta payloads, if modified.

The response to a POST operation MUST include any properties not set to their default value, and the response to a PUT/PATCH operation MUST include any properties whose values were changed as part of the operation.

The omit-values preference does not affect a request payload. 

This PR is used to discuss the service side support for omit_values prefer header.