Respect numerical keys in `add_query_arg()`, use `array_replace()` instead of `array_merge()`.

Adds unit test.

Props tyxla.
Fixes #31306.

Built from https://develop.svn.wordpress.org/trunk@31966


git-svn-id: http://core.svn.wordpress.org/trunk@31945 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -782,7 +782,7 @@ function add_query_arg() {
 	$qs = urlencode_deep( $qs ); // this re-URL-encodes things that were already in the query string
 	if ( is_array( $args[0] ) ) {
 		$kayvees = $args[0];
-		$qs = array_merge( $qs, $kayvees );
+		$qs = array_replace( $qs, $kayvees );
 	} else {
 		$qs[ $args[0] ] = $args[1];
 	}

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-beta3-31965';
+$wp_version = '4.2-beta3-31966';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.