Reset Current.site between tests

In order to prevent test pollution when a previous test sets
`Current.site` to a value that differs from `Site.first`,
which could cause `Language.default` to return `nil` and
trigger uniqueness conflicts in factories.

Author: tvdeyen

spec/rails_helper.rb

@@ -82,8 +82,8 @@
 
   config.use_transactional_fixtures = true
 
-  # All specs are running in transactions, but feature specs not.
   config.before(:each) do
+    Alchemy::Current.site = nil
     ::I18n.locale = :en
   end