Adds schema parameter into enable_extension

This patch tries to solve Heroku's new [PostgreSQL extension policy](https://devcenter.heroku.com/changelog-items/2446)
while keeping the migration and schema code idiomatic.

PostgreSQL adapter method `enable_extension` now allows to add an schema in its name.
The extension must be installed on another schema.

Usage:

`enable_extension('other_schema.hstore')`

The `enable_extension` can work with `schema` only if the given schema
already exists in the database.

activerecord/CHANGELOG.md

@@ -1,3 +1,11 @@
+*   PostgreSQL adapter method `enable_extension` now allows parameter to be `[schema_name.]<extension_name>`
+    if the extension must be installed on another schema.
+
+    Example: `enable_extension('heroku_ext.hstore')`
+
+    *Leonardo Luarte*
+
+
 *   `ActiveRecord::Relation`’s `#any?`, `#none?`, and `#one?` methods take an optional pattern
     argument, more closely matching their `Enumerable` equivalents.
 

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -442,9 +442,11 @@ module ActiveRecord
       end
 
       def enable_extension(name, **)
-        exec_query("CREATE EXTENSION IF NOT EXISTS \"#{name}\"").tap {
-          reload_type_map
-        }
+        schema, name = name.to_s.split(".").values_at(-2, -1)
+        sql = +"CREATE EXTENSION IF NOT EXISTS \"#{name}\""
+        sql << " SCHEMA #{schema}" if schema
+
+        exec_query(sql).tap { reload_type_map }
       end
 
       # Removes an extension from the database.

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

@@ -17,6 +17,12 @@ class PostgresqlExtensionMigrationTest < ActiveRecord::PostgreSQLTestCase
     end
   end
 
+  class EnableHstoreInSchema < ActiveRecord::Migration::Current
+    def change
+      enable_extension "other_schema.hstore"
+    end
+  end
+
   def setup
     super
 
@@ -50,6 +56,19 @@ class PostgresqlExtensionMigrationTest < ActiveRecord::PostgreSQLTestCase
     assert @connection.extension_enabled?("hstore"), "extension hstore should be enabled"
   end
 
+  def test_enable_extension_migration_with_schema
+    @connection.disable_extension("hstore")
+    @connection.create_schema "other_schema"
+
+    migrations = [EnableHstoreInSchema.new(nil, 1)]
+    ActiveRecord::Migrator.new(:up, migrations, @connection.schema_migration, @connection.internal_metadata).migrate
+
+    assert @connection.extension_enabled?("hstore"), "extension hstore should be enabled"
+  ensure
+    @connection.drop_schema "other_schema", if_exists: true
+  end
+
+
   def test_disable_extension_migration_ignores_prefix_and_suffix
     @connection.enable_extension("hstore")