Add test covering unpacking 0-length rdatas
Make TestDynamicUpdateZeroRdataUnpack pass

Straight up: took a guess and the tests pass.
Test that Msg.RemoveRRset() works as intended
Make RemoveRRset work as intended
Lose a loop in Msg.RemoveRRset() and use copyHeader()

Per @miekg's advice
Merge pull request #154 from andrewtj/atj-update-misc

DNS Update Misc