Returns false if the requested resource hasn't changed, true
otherwise. This method is called by the cache system to
ensure the validity of the cached response. It is the
producer responsibility to provide the fastest possible
implementation of this method or, whether this is not
possible and the costs of the change evaluation is
comparable to the production costs, to return
true directly with no further delay, thus reducing
the evaluation overhead to a minimum.
This method is guaranteed to be called after at least
a single call to any production methods getStream or getDocument.
The context is the trigger of the changeable point and may
differ between implementations.