test case for custom PostgreSQL enum type.

This test currently outputs the following warning:

```
unknown OID: current_mood(3567879) (SELECT  "postgresql_enums".* FROM "postgresql_enums"   ORDER BY "postgresql_enums"."id" ASC LIMIT 1)
unknown OID: current_mood(3567879) (SELECT  "postgresql_enums".* FROM "postgresql_enums"  WHERE "postgresql_enums"."id" = $1 LIMIT 1)
```

We have an open PR to deal with this issue. It will dynamically
register the OID for enum columns. This test case is merely to exhibit
the current behavior of PostgreSQL enum columns.

activerecord/test/cases/adapters/postgresql/enum_test.rb

+# -*- coding: utf-8 -*-
+require "cases/helper"
+require 'active_record/base'
+require 'active_record/connection_adapters/postgresql_adapter'
+
+class PostgresqlEnumTest < ActiveRecord::TestCase
+  class PostgresqlEnum < ActiveRecord::Base
+    self.table_name = "postgresql_enums"
+  end
+
+  def teardown
+    @connection.execute 'DROP TABLE IF EXISTS postgresql_enums'
+    @connection.execute 'DROP TYPE IF EXISTS mood'
+  end
+
+  def setup
+    @connection = ActiveRecord::Base.connection
+    @connection.transaction do
+      @connection.execute <<-SQL
+        CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');
+      SQL
+      @connection.create_table('postgresql_enums') do |t|
+        t.column :current_mood, :mood
+      end
+    end
+  end
+
+  def test_enum_mapping
+    @connection.execute "INSERT INTO postgresql_enums VALUES (1, 'sad');"
+    enum = PostgresqlEnum.first
+    assert_equal "sad", enum.current_mood
+
+    enum.current_mood = "happy"
+    enum.save!
+
+    assert_equal "happy", enum.reload.current_mood
+  end
+end