reference: allow 'force' in .create to be truthy

While looking at the checkout PR, I saw a couple of `Reference.create` calls and I had to check the docs about the third argument. Allowing non boolean values can make those a bit more readable.

```ruby
 Rugged::Reference.create(@repo,
                             "refs/heads/unit_test",
                             "refs/heads/master",
                             :force)
```