Tiny improvement on render() method in Magento\Backend\Block\Widget\Grid\Column\Ren...

...derer\Concat which allows now calls to any method on a model. Which is a huge win!

Configuration in adminhtml grid can look like this:

```php
        $this->addColumn('product_price', array(
            'header'    => __('RRP incl. Tax<br>Sales Prices incl. Tax'),
            'getter'    => ['returnProductPrice', 'getProductFinalPrice'],
            'align'     => 'right',
            'type'  => 'concat',
            'separator' => '<br>',
        ));
```

or this

```php
        $this->addColumn('product_price2', array(
            'header'    => __('RRP incl. Tax<br>Sales Prices incl. Tax'),
            'index'    => ['product_price', 'product_final_price'],
            'align'     => 'right',
            'type'  => 'concat',
            'separator' => '<br>',
        ));
```

No tests added ...

Thanks for merging :-)
True that method_exists() is faster than is_callable(). However, is_callable() will be able to correctly recognise method calls handled by __call() in PHP 5-7, while method_exists() will not.