Fixed serialisation of non-dynamic ArrayPlugs.

We were serialising a construct/addChild pair for the first element of the array, even though the element would be provided again via the constructor next time. This led to one additional plug being added each time the ArrayPlug was serialised and unserialised.

Fixes #580.
Merge pull request #868 from johnhaddon/arrayPlugSerialisationFix

Fixed serialisation of non-dynamic ArrayPlugs.