Update Opengraph.php

When running multiple url's after each other, the storage isn't reset; 
```
$data = array();
$reader = new Opengraph\Reader();
$reader->parse(file_get_contents('http://www.imdb.com/title/tt0117500/'));
$data[] = $reader->getArrayCopy();
$reader = new Opengraph\Reader();
$reader->parse(file_get_contents('http://nu.nl/'));
$data[] = $reader->getArrayCopy();
```
When the second URL doesn't have Opengraph data, it shows the data of the first URL.