[ruby/resolv] Fix confusion of received response message

	This is a follow up for commit 33fb966197f1 ("Remove sender/message_id
	pair after response received in resolv", 2020-09-11).

	As the @senders instance variable is also used for tracking transaction
	ID allocation, simply removing an entry without releasing the ID would
	eventually deplete the ID space and cause
	Resolv::DNS.allocate_request_id to hang.

	It seems the intention of the code was to check that the received DNS
	message is actually the response for the question made within the method
	earlier. Let's have it actually do so.

	[Bug #12838] https://bugs.ruby-lang.org/issues/12838
	[Bug #17748] https://bugs.ruby-lang.org/issues/17748

	https://github.com/ruby/resolv/commit/53ca9c9209
	---
	 lib/resolv.rb | 6 +++---
	 1 file changed, 3 insertions(+), 3 deletions(-)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67946 b2dd03c8-39d4-4d8f-98ff-823fe69b080e