Keep backwards compatibility in kill api.

Return an error when the container is stopped only in api versions
equal or greater than 1.20 (docker 1.8).

Signed-off-by: David Calavera <david.calavera@gmail.com>

integration-cli/docker_cli_kill_test.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"fmt"
+	"net/http"
 	"strings"
 
 	"github.com/go-check/check"
@@ -87,3 +89,12 @@ func (s *DockerSuite) TestKillWithInvalidSignal(c *check.C) {
 		c.Fatal("Container should be in running state after an invalid signal")
 	}
 }
+
+func (s *DockerSuite) TestKillofStoppedContainerAPIPre120(c *check.C) {
+	dockerCmd(c, "run", "--name", "docker-kill-test-api", "-d", "busybox", "top")
+	dockerCmd(c, "stop", "docker-kill-test-api")
+
+	status, _, err := sockRequest("POST", fmt.Sprintf("/v1.19/containers/%s/kill", "docker-kill-test-api"), nil)
+	c.Assert(err, check.IsNil)
+	c.Assert(status, check.Equals, http.StatusNoContent)
+}