Allow to update a resource without retreiving it first

Rationale:

  Sometime you don't care about the current state of a resource
  you just want to update one of it's attributes.
  It should only require one request.

fixes #52

```ruby
  c = Stripe::Customer.new("cus_1EqKjPaFs4ZwDD")
  c.description = 'Ny new Description'
  c.save
```

Before:

```json
{
  error: {
    type: "invalid_request_error",
    message: "A parameter provided in the URL (id) was repeated as a GET or POST parameter. You can only provide this information as a portion of the URL.",
    param: "id",
  }
}
```

After:

Successfully update the customer and return it's whole state.