REST API: Site Settings: Add support for "twitter_via" site setting

Related: Automattic/calypso-pre-oss#3545

Merges r112065-wpcom.

Author: Andrew Duthie

json-endpoints/class.wpcom-json-api-site-settings-endpoint.php

@@ -22,7 +22,7 @@ function callback( $path = '', $blog_id = 0 ) {
 		if ( defined( 'IS_WPCOM' ) && IS_WPCOM ) {
 			$this->load_theme_functions();
 		}
-		
+
 		if ( ! is_user_logged_in() ) {
 			return new WP_Error( 'Unauthorized', 'You must be logged-in to manage settings.', 401 );
 		} else if ( ! current_user_can( 'manage_options' ) ) {
@@ -153,6 +153,7 @@ public function get_settings_response() {
 					'disabled_likes'          => (bool) get_option( 'disabled_likes' ),
 					'disabled_reblogs'        => (bool) get_option( 'disabled_reblogs' ),
 					'jetpack_comment_likes_enabled' => (bool) get_option( 'jetpack_comment_likes_enabled', false ),
+					'twitter_via'             => (string) get_option( 'twitter_via' ),
 				);
 
 				if ( class_exists( 'Sharing_Service' ) ) {