JSON snake case

For the active model adapter, building JSON to mock HTTP request is not working as Factory-Guy serializes the model attributes using camel case (instead of snake case) and the active model adapter does not take care about camel case.

For example ```FactoryGuy.build('customer')``` returns something like ```{customerNumber: ..., customerContact: ...}``` for the factory

```
FactoryGuy.define('customer', {
  default: {
    customerNumber: ...,
    customerContact: ...,
  }
});
```

But to fit the requirements of the active model adapter, the JSON to mock the HTTP request has to be ```{customer_number: ..., customer_contact: ...}```.