Fix docstring being inconsistent with behaviour

The docstring for missing in _SchemaNode says:

> When `missing` is `colander.drop`, the node is dropped from the schema if it isn't set during serialization/deserialization.

In reality it is only dropped on deserialization.
The attached commit changes the return value of `_SchemaNode.serialize` to `colander.drop` if `_SchemaNode.missing` is `colander.drop` and the cstruct return from the type serialization method is `colander.null`.

I may have misinterpreted the docstring. If that is the case, I think it could be made slightly clearer.