disable simplepie caching, see https://github.com/simplepie/simplepie/issues/345#issuecomment-34718099

fetcher/feedfetcher.php

@@ -82,6 +82,8 @@ class FeedFetcher implements IFeedFetcher {
 		$simplePie = $this->simplePieFactory->getCore();
 		$simplePie->set_feed_url($url);
 		$simplePie->enable_cache(true);
+		$simplePie->set_stupidly_fast(true);  // disable simple pie sanitation
+		                                      // we use htmlpurifier
 		$simplePie->set_timeout($this->fetchTimeout);
 		$simplePie->set_cache_location($this->cacheDirectory);
 		$simplePie->set_cache_duration($this->cacheDuration);

tests/unit/fetcher/FeedFetcherTest.php

@@ -140,6 +140,9 @@ class FeedFetcherTest extends \OCA\AppFramework\Utility\TestUtility {
 		$this->core->expects($this->once())
 			->method('set_cache_location')
 			->with($this->equalTo($this->cacheDirectory));
+		$this->core->expects($this->once())
+			->method('set_stupidly_fast')
+			->with($this->equalTo(true));
 		$this->core->expects($this->once())
 			->method('set_cache_duration')
 			->with($this->equalTo($this->cacheDuration));