Workaround for a PHP/libxml-bug with entities

As described on http://bugs.php.net/bug.php?idE996, current versions of libxml (2.7.0/1) lead to the fact that the xml parser of PHP eats predefined entities. As proposed on http://bugs.simplepie.org/issues/show/101 predefined entities are replaced by their numeric equivalents. There is no condition in this patch as there are people reporting it isn't gone in 2.7.2, i can confirm that, and furthermore here PHP is reporting libxml 20632 and the bug exists, too (it is linked to libxml2 though which has version 2.7.2). As soon as there is definite knowledge on which versions are actually concerned and how to detect them in PHP a condition should be added as this patch might decrease the performance of the affected functions.

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