Do not rely on SimplePie escaping

Revert "No need to purifier content coming from SimplePie since it is already sanitized (Can we trust it?)"

This reverts commit d9f22866d33dd6fc8c420337fc569444d7824160.

As reported by Thorsten Glaser, we cannot rely on SimplePie escaping.

http://lists.planetforge.org/pipermail/discussions/2012-December/000235.html
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