Previously, the RB.BaseResource model was using an object literal for

its defaults, but its default value for extra data was an empty object.

This could possibly result in shared state if a resource did not return

an extra_data JSON field, so the defaults property has been changed

into a function to avoid this.
All child resources of RB.BaseResource have been modified to use a

function instead of an attribute for defaults and to correctly invoke

the parent class's definition of defaults.

Ran JS tests.