Fix order dependency in site_setting_spec.rb

The spec contained an order dependency which would cause the default
bool test to fail. You can confirm this by running the spec with the
option "--order rand" a couple times. This dependency was caused by
surprising behavior in SiteSetting::setting as shown below:

```ruby
SiteSetting.setting(:bool_setting?, false)
SiteSetting.refresh!
SiteSetting.bool_setting? #=> false
SiteSetting.bool_setting = true
SiteSetting.bool_setting? #=> true
SiteSetting.setting(:bool_setting?, false)
SiteSetting.refresh!
SiteSetting.bool_setting? #=> true!
```

The spec is merely desriptive, and I have labeled what I think could be
possible bugs.