Default value for $credentials parameter in wp_signon() should be an array, not a string.

props tivnet.
fixes #27130.

git-svn-id: https://develop.svn.wordpress.org/trunk@27350 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/user.php

@@ -22,7 +22,7 @@
  * @param bool $secure_cookie Optional. Whether to use secure cookie.
  * @return object Either WP_Error on failure, or WP_User on success.
  */
-function wp_signon( $credentials = '', $secure_cookie = '' ) {
+function wp_signon( $credentials = array(), $secure_cookie = '' ) {
 	if ( empty($credentials) ) {
 		if ( ! empty($_POST['log']) )
 			$credentials['user_login'] = $_POST['log'];