Actor uses animation from <skin> instead of <animation>

[chapulina]

On Gazebo-classic, this actor would have a brown shirt (from moonwalk.dae) and do an walking animation (from walk.dae):

<actor name="actor_walking">
    <skin>
        <filename>file://media/models/moonwalk.dae</filename>
        <scale>1.0</scale>
    </skin>
    <animation name="walk">
        <filename>file://media/models/walk.dae</filename>
    </animation>
</actor>

On Ignition, this equivalent actor is using the correct skin with a brown shirt, but it's using the moonwalk animation instead of walkiing:

<actor name="actor_walking">
    <skin>
        <filename>https://fuel.ignitionrobotics.org/1.0/Mingfei/models/actor/tip/files/meshes/moonwalk.dae</filename>
        <scale>1.0</scale>
    </skin>
    <animation name="walk">
        <filename>https://fuel.ignitionrobotics.org/1.0/Mingfei/models/actor/tip/files/meshes/walk.dae</filename>
    </animation>
</actor>

I believe we're storing the animation from the skin and using that by default even if there's an <animation> tag. We should do like Gazebo-classic and don't ignore <animation>.

[mohamedsayed18]

if a trajectory is added it works fine as in ignitionrobotics/docs#43