Rewrite: Ensure `WP_Rewrite::flush_rules()` does not delete the 'rewrite_rules' option.

Instead, the option gets updated to an empty string.
Adds unit tests.

Props SergeyBiryukov, jesin, voldemortensen.
Fixes #29107.
Built from https://develop.svn.wordpress.org/trunk@36254


git-svn-id: http://core.svn.wordpress.org/trunk@36221 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.5-alpha-36253';
+$wp_version = '4.5-alpha-36254';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.