object notation should be optional
hi,

when i'm trying to generate a type and a mutation from this example schema, the generator will crashes because of missing 'type' property on the field.

```js
'use strict'

var mongoose = require('mongoose')
var example= new mongoose.Schema({
	title: String
})

module.exports = mongoose.model('Example', example)
```

changing the schema in this way make the generator works again:

```js
'use strict'

var mongoose = require('mongoose')
var example= new mongoose.Schema({
	title: { type: String, }
})

module.exports = mongoose.model('Example', example)
```