`Dry::Struct::Error` when serializing document sections list

[lannon]

The PandaDoc API does not return full DocumentSection objects when listing document sections for a document. Instead, it returns an abridged object containing just uuid and name attributes. We should make all other attributes on the DocumentSection object optional since they're not guaranteed to be present. This follows the existing pattern for Document objects.

[opti]

Fixed by #63

Thanks for the fix @lannon