* Add regression test for broken DNS truncation

With the latest changes to the implementation, request.Scrub() returns
invalid response messages which are larger than the advertised buffer
size. This commit adds a failing test for that.

* Add benchmark for response truncation

* request: re-adjust size for edns0 RR

Cherry-picked the test and benchmark commits from #1962 and make the
failing test not fail, by adding minimal fix in request.go.

This makes size 12 bytes smaller if we need to add back the OPT RR in
the message.

Closes: #1962
Signed-off-by: Miek Gieben <miek@miek.nl>

* Scrub: correctly check for EDNS0, not only Do

Make Scrub check for EDNS0, not only if the Do bit is set.
Signed-off-by: Miek Gieben <miek@miek.nl>