Remove sender/message_id pair after response received in resolv

	Once a response for a given DNS request has been received (which
	requires a matching message id), the [sender, message_id] pair
	should be removed from the list of valid senders.  This makes it
	so duplicate responses from the same sender are ignored.

	Fixes [Bug #12838]
	---
	 lib/resolv.rb           |   2 +-
	 test/resolv/test_dns.rb | 113 ++++++++++++++++++++++++++++++++++++++++++++++++
	 2 files changed, 114 insertions(+), 1 deletion(-)