title: dynamic body with segment shape do not move with a joint
body: <p>In pymunk, I set the body mass and the space gravity, it should automatic fall down, but the body with segment shape do not move, here is my code in pyglet</p>
  
  <pre><code>import pyglet
  import pymunk
  from pymunk.pyglet_util import DrawOptions
  
  window = pyglet.window.Window(1280,720,resizable=False)
  options = DrawOptions()
  
  space = pymunk.Space()
  space.gravity = 0,-1000
  
  ball_mass = 1
  ball_radius = 10
  
  ball_moment = pymunk.moment_for_circle(ball_mass,0,10)
  ball = pymunk.Body(ball_mass,ball_moment)
  ball_shape = pymunk.Circle(ball,ball_radius)
  ball.position =200,500
  ball_shape.elasticity = 1
  ball_shape.friction = 1
  
  space.add(ball,ball_shape)
  
  stick = pymunk.Body(1,100,body_type=pymunk.Body.DYNAMIC)
  stick_shape = pymunk.Segment(stick,(0,0),(150,150),4)
  stick.position = (300,400)
  pin = pymunk.PivotJoint(space.static_body,stick,(300,400))
  # stick.apply_impulse_at_local_point((0,-100),(150,150))
  space.add(stick,stick_shape,pin)
  
  
  @window.event
  def on_draw():
      window.clear()
      space.debug_draw(options)
  
  def update(dt):
      space.step(dt)
  
  
  if __name__ == '__main__':
      pyglet.clock.schedule_interval(update, 1/60.0)
      pyglet.app.run()
  </code></pre>
  
  <p>the ball fall down but the stick do not move, or apply_impulse_at_local_point on stick, I just wondering, if there is no PivotJoint, the stick fall down, I just pin one point of the stick, why it does not move, it should rotate, is it?</p>