* server: actually scrub response

Did all the worked, hooked it up wrongly :(

This also needs test, but those are hard(er) because we only receive
packets after they have been decoded; i.e. we never see the wirefmt.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Add tests

Add a test for checking is compression pointers are set in the packet.
This also adds an undocumented 'large' feature to the erratic plugin to
send large responses that should be compressed.

Commenting the Scrub out in server results in:

=== RUN   TestCompressScrub
--- FAIL: TestCompressScrub (0.00s)
    compression_scrub_test.go:41: Expected returned packet to be < 512, got 839
FAIL
exit status 1
FAIL    github.com/coredns/coredns/test 0.036s

Actually checking the size might be easier, but lets be thorough here
and check the pointers them selves.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Fix tests
Signed-off-by: Miek Gieben <miek@miek.nl>

* plugin erratic: fix e.large

always put an rr in the reply, fix e.large in erractic and add test to
check for it.
Signed-off-by: Miek Gieben <miek@miek.nl>