mixed dimmentionality in ST_AsTWKB gives strange results

[nicklas]

ST_AsTWKB version that takes input from array of geometries have no check for mixed dimensionality.

In theory when returning a collection type (7) it would be possible to handle mixed dimensionality. But since all geoemtries is transportet internally as geometry collection it doesn't work.

And the very first byte in the twkb will then be inconsistent with the subgeoemtries

[nicklas]

For now I think the right way is to not allow mixed dimensionality.

That is the same behavior as ST_Collect

Done in r13731