Fix loading document type attributes from DynamoDB when when saveUnknown=true

[jonathonlui]

This PR fixes an issue when using saveUnknown + useDocumentTypes and trying to retrieve a model that hasn't yet been "used" for example:

  var Cat = new Schema({
    id: Number,
  }, {
    useDocumentTypes: true,
    saveUnknown: true,
  });

  // Assume an item is in the Cat table:
  // {
  //   id: { N: '1' }
  //   petProperties: {
  //     M: {
  //       name: { S: 'Fluffy' },
  //       age: { N: 10 },
  //     }
  //   }
  // }

  Cat.get({ id: 1}};

  // throws SyntaxError: Unexpected token o in JSON at position 1

This is because since the Cat schema doesn't have getProperties attribute Schema.parseDynamo will create an new attribute with an Object type because Attribute.lookupType returns Object for Dynamo "'M" types.

This then causes the Attribute.parseDynamo to be parsed using JSON.parse instead of the correct mapify.

Interestingly this error doesn't happen if the model has been used to create (or put/save) before doing get/query/scan.

  var Cat = new Schema({
    id: Number,
  }, {
    useDocumentTypes: true,
    saveUnknown: true,
  });

  Cat.create({
    id : 1,
    petProperties: {
      name: 'Fluffy',
      age: 10,
    },
  );

  // Cat's schema now has a petProperties attribute

  Cat.get({ id: 1}};

  // No error

This is because when Cat.create runs, it creates the petProperties attribute in the model's schema, and when Cat.get runs, Schema.parseDynamo see that is has petProperties and doesn't try to create a new, incorrectly-typed Attribute.

[fishcharlie]

LGTM

[fishcharlie]

Thanks so much for this @jonathonlui. I'm going to have @brandongoode review this at some point before we get it merged in. We will aim to get this in version 0.9. Thanks again!