🐛 Fix closed check when handling querySubscribe()

The `Agent` attempts to check if it's been closed when getting the
response for a query subscribe.

However, it incorrectly tries to access `this` inside a `function`,
which doesn't give the correct value, and in `strict mode`, will
actually result in an error, since `this` will be `undefined`.

This change adds a test for this case and fixes it.

lib/agent.js

@@ -569,7 +569,7 @@ Agent.prototype._querySubscribe = function(queryId, collection, query, options,
   }
   this.backend.querySubscribe(this, collection, query, options, function(err, emitter, results, extra) {
     if (err) return finish(err);
-    if (this.closed) return emitter.destroy();
+    if (agent.closed) return emitter.destroy();
 
     agent._subscribeToQuery(emitter, queryId, collection, query);
     // No results are returned when ids are passed in as an option. Instead,

test/client/query-subscribe.js

@@ -729,6 +729,29 @@ function commonTests(options) {
     connection.get('dogs', 'fido').on('error', done).create({age: 3});
   });
 
+  it('does not reply if the agent is closed before the query returns', function(done) {
+    var backend = this.backend;
+    var connection = backend.connect();
+    var agent = connection.agent;
+
+    backend.use('query', function(request, next) {
+      backend.use('reply', function() {
+        done(new Error('unexpected reply'));
+      });
+
+      expect(agent.closed).to.be.false;
+      agent.stream.on('close', function() {
+        expect(agent.closed).to.be.true;
+        next();
+        done();
+      });
+
+      agent.close();
+    });
+
+    connection.createSubscribeQuery('dogs', {});
+  });
+
   describe('passing agent.custom to the DB adapter', function() {
     var connection;
     var expectedArg = {