add shutdown context unit test

http/download_test.go

@@ -154,3 +154,13 @@ func (s *DownloaderSuite) TestGetLengthConnectError(c *C) {
 
 	c.Assert(err, ErrorMatches, ".*no such host")
 }
+
+func (s *DownloaderSuite) TestContextCancel(c *C) {
+	ctx, cancel := context.WithCancel(s.ctx)
+	s.ctx = ctx
+
+	cancel()
+	_, err := s.d.GetLength(s.ctx, "http://nosuch.host.invalid./")
+
+	c.Assert(err, ErrorMatches, ".*context canceled.*")
+}