Define enable_extension method to prevent undefined method error

When mixing postgresql and another adapter like sqlite3 (for dev and test
respectively), the task `db:test:prepare` will fail due to the `enable_extension`
method not being defined in the abstract adapter. This patch simply adds an
empty definition to prevent it.

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -293,6 +293,14 @@ module ActiveRecord
         false
       end
 
+      # This is meant to be implemented by the adapters that support extensions
+      def disable_extension(name)
+      end
+
+      # This is meant to be implemented by the adapters that support extensions
+      def enable_extension(name)
+      end
+
       # A list of extensions, to be filled in by adapters that support them. At
       # the moment only postgresql does.
       def extensions

activerecord/test/cases/adapters/mysql/mysql_adapter_test.rb

@@ -108,6 +108,18 @@ module ActiveRecord
         assert_equal 2, result.column_types['status'].type_cast(result.last['status'])
       end
 
+      def test_supports_extensions
+        assert_not @conn.supports_extensions?, 'does not support extensions'
+      end
+
+      def test_respond_to_enable_extension
+        assert @conn.respond_to?(:enable_extension)
+      end
+
+      def test_respond_to_disable_extension
+        assert @conn.respond_to?(:disable_extension)
+      end
+
       private
       def insert(ctx, data, table='ex')
         binds   = data.map { |name, value|

activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb

@@ -354,6 +354,18 @@ module ActiveRecord
         assert_nil @conn.primary_key('failboat')
       end
 
+      def test_supports_extensions
+        assert_not @conn.supports_extensions?, 'does not support extensions'
+      end
+
+      def test_respond_to_enable_extension
+        assert @conn.respond_to?(:enable_extension)
+      end
+
+      def test_respond_to_disable_extension
+        assert @conn.respond_to?(:disable_extension)
+      end
+
       private
 
       def assert_logged logs