Colander missing=drop + Sphinx (via cornice-autodoc) result in 'not JSON serializable' while building docs

[nphilipp]

This is essentially this issue: Cornices/cornice#315

I just ran into this when I tried to add this node to a (mapping) schema:

    from_tag = colander.SchemaNode(
        colander.String(),
        missing=colander.drop,
    )

For the time being, I've replaced it with missing=None because the code seems to cope with it, but as it's an optional thing it'd feel more natural if I could check for its presence rather than its null-ness.