Use object held by form builder when available

This allows `PropertySetFormBuilder` to use the object passed into `form_for` if it exists. If not, it falls back to trying to fetch it using `instance_variable_get` as before.

```ruby
form_for(current_account, ...) do |f|
  f.property_set(:settings).hidden_field :blah
end
```
**Before:**

The above would always try to fetch `@account` even though the form builder had a reference to it already.

**Now:**

It uses the passed-in account object, which removes the need to set up instance variables just to make `property_sets` happy.