Because we can not provide defaults for all types, .get() might trigger
VCL failure now.

Note that the interface is fully compatible with existing VCL, but while
previously a missing fallback argument would trigger a compile time
error, it will result in a VCL failure now.

The types without a default which might trigger a failure are:

* ACL: Trying to match a NULL ACL would trigger a failure in
  varnish-cache anyway

* BODY: Due to the lack of examples, it is unclear what a NULL body
  would look like

  A sensible default might be added later (maybe the empty body?)

* HEADER: A default makes no sense