Merge pull request #2789 from ojiry/support_oracle

Fix #encoding error by oracle_enhanced

Author: mshibuya

lib/rails_admin/adapters/active_record.rb

@@ -72,6 +72,8 @@ def encoding
           ::ActiveRecord::Base.connection.select_one("SELECT ''::text AS str;").values.first.encoding
         when 'mysql2'
           ::ActiveRecord::Base.connection.instance_variable_get(:@connection).encoding
+        when 'oracle_enhanced'
+          ::ActiveRecord::Base.connection.select_one("SELECT dummy FROM DUAL").values.first.encoding
         else
           ::ActiveRecord::Base.connection.select_one("SELECT '' AS str;").values.first.encoding
         end