Cast boolean values to 0/1 in hidden fields

This allows a `hidden_field` tag to "just work" even when the value being persisted is a boolean.

Given:
```ruby
property_set :settings do
  property :sso_agent_auth, :type => :boolean, :default => true
end
```

then the following:

```
<%= f.property_set(:settings).hidden_field :sso_agent_auth %>
```

yields:

`<input id="account_settings_sso_agent_auth" name="account[settings][sso_agent_auth]" type="hidden" value="1">`

@morten @staugaard @kbuckler 

cc/ @rapheld