Description:
------------
Using mysql 5.5.19, loaded auth_socket plugin and created a user to auth with it:

mysql> INSTALL PLUGIN auth_socket SONAME 'auth_socket.so';
mysql> CREATE USER 'someuser'@'localhost' IDENTIFIED WITH auth_socket;

Also assume an existing database 'website'.

Trying to auth with user someuser works, when using the mysql client, but fails with mysqlnd as shown below.

Test script:
---------------
<?php
$mysqli = new mysqli("localhost", "someuser", "", "website");

if (mysqli_connect_errno()) {
        printf("Connect failed: %s\n", mysqli_connect_error());
            exit();
}

printf("Host information: %s\n", $mysqli->host_info);

$mysqli->close();
?>

Expected result:
----------------
Host information: Localhost via UNIX socket

Actual result:
--------------
PHP Warning:  mysqli::mysqli(): (08004/1251): Client does not support authentication protocol requested by server; consider upgrading MySQL client in /home/someuser/mysql.php on line 2
PHP Stack trace:
PHP   1. {main}() /home/someuser/mysql.php:0
PHP   2. mysqli->mysqli() /home/someuser/mysql.php:2
Connect failed: Client does not support authentication protocol requested by server; consider upgrading MySQL client