String values converted to date even when type is string

[KyleLilly]

The castValues function is converting string data that is parseable as a date via moment.js ie 20000101 to a date and returning a date object even when the model defines the column as a string:

attributes: {
  version: {
    type: "string",
    primaryKey: true,
    required: true
  },
  description: {
    type: "string"
  }
}

The issue really becomes a problem because moment.isValid() returns true for something like 20000101 but after running the replace on the value which is a string it's still a string and doing new Date(stripped).valueOf() results in NaN because the value "20000101" is not valid for a Date.

Ideally castValues would just return the string if the model type is string and if it's a date then moment should be used to do the parsing.

I can send a PR if you want.