Preserve layer options interactivity in named maps

[tylerparsons]

It looks like the interactivity property of named maps, which stores a list of all columns that a particular layer uses in it's click and hover tooltips, was not getting copied to redis when named maps were created or updated. This change is to ensure the property is always copied on a create/update of a named map.

The cause of this seems to be a change in the schema of the layers.options object, which is a big json object created on the frontend when creating maps. What likely happened is that the carto builder introduced a schema change which was not replicated in our editor, and the old schema was no longer maintained by the backend.

[tylerparsons]

@gfiorav the logic has changed somewhat with the new update, but it's actually closer to what was originally there. In short, the impact of this PR is to make the default interactivity equal to 'cartodb_id', as opposed to an empty string.

I added a new spec to ensure that interactivity gets this default when doesn't contain tooltips or infowindows.