ApcProxy::apc_add functionality is incorrect

[fyrye]

Since apc_add and apc_store can accept a string or array for the key, when a string is supplied a var must also be applied, otherwise when key is an array it is handled as a key:var paired array

$cache->apc_add(['Hello' => 'World']); results in an exception being thrown.

public function apc_add($key, $var = null, $ttl = 0)
    {
        if (is_array($key) && $var === null) {
            throw new \InvalidArgumentException('When $key is set $var cannot be null');
        }

should be something like

public function apc_add($key, $var = null, $ttl = 0)
    {
        if (is_string($key) && $var === null) {
            throw new \InvalidArgumentException('When $key is set $var cannot be null');
        } elseif (is_array($key) && $var !== null) {
            throw new \InvalidArgumentException('When $key is an array $var must be null');
        }

reference: http://php.net/manual/en/function.apcu-add.php

[gordalina]

Very nice work debugging this, can you submit a pull request for this change?