Notice in Zencoder::update_attributes()

The following line appears in update_attributes():

``` php
} elseif (!function_exists($this->$attr_name)) {
```

I think the goal of this statement is to check if there is a method named `$attr_name` on `$this`. This should be written as:

``` php
} elseif (!method_exists($this, $attr_name)) {
```
