Plugin_Upgrader::upgrade( string $plugin, array $args = array() )

	public function upgrade( $plugin, $args = array() ) {

		$defaults = array(
			'clear_update_cache' => true,
		);
		$parsed_args = wp_parse_args( $args, $defaults );

		$this->init();
		$this->upgrade_strings();

		$current = get_site_transient( 'update_plugins' );
		if ( !isset( $current->response[ $plugin ] ) ) {
			$this->skin->before();
			$this->skin->set_result(false);
			$this->skin->error('up_to_date');
			$this->skin->after();
			return false;
		}

		// Get the URL to the zip file
		$r = $current->response[ $plugin ];

		add_filter('upgrader_pre_install', array($this, 'deactivate_plugin_before_upgrade'), 10, 2);
		add_filter('upgrader_clear_destination', array($this, 'delete_old_plugin'), 10, 4);
		//'source_selection' => array($this, 'source_selection'), //there's a trac ticket to move up the directory for zip's which are made a bit differently, useful for non-.org plugins.
		if ( $parsed_args['clear_update_cache'] ) {
			// Clear cache so wp_update_plugins() knows about the new plugin.
			add_action( 'upgrader_process_complete', 'wp_clean_plugins_cache', 9, 0 );
		}

		$this->run( array(
			'package' => $r->package,
			'destination' => WP_PLUGIN_DIR,
			'clear_destination' => true,
			'clear_working' => true,
			'hook_extra' => array(
				'plugin' => $plugin,
				'type' => 'plugin',
				'action' => 'update',
			),
		) );

		// Cleanup our hooks, in case something else does a upgrade on this connection.
		remove_action( 'upgrader_process_complete', 'wp_clean_plugins_cache', 9 );
		remove_filter('upgrader_pre_install', array($this, 'deactivate_plugin_before_upgrade'));
		remove_filter('upgrader_clear_destination', array($this, 'delete_old_plugin'));

		if ( ! $this->result || is_wp_error($this->result) )
			return $this->result;

		// Force refresh of plugin update information
		wp_clean_plugins_cache( $parsed_args['clear_update_cache'] );

		return true;
	}