Multisite: Set `WP_Network` `blog_id` property default to string as expected.

The `blog_id` property is always returned (and expected) as a string, so we should set it as one by default.

Props flixos90.
See #36717.

Built from https://develop.svn.wordpress.org/trunk@37871


git-svn-id: http://core.svn.wordpress.org/trunk@37812 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-network.php

@@ -63,7 +63,7 @@ class WP_Network {
 	 * @access private
 	 * @var string
 	 */
-	private $blog_id = 0;
+	private $blog_id = '0';
 
 	/**
 	 * Domain used to set cookies for this network.

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.6-alpha-37870';
+$wp_version = '4.6-alpha-37871';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.