[FreshRSS] decode special chars for MAYBE_HTML
[FreshRSS] decode special chars for MAYBE_HTML

Patch for https://github.com/simplepie/simplepie/issues/350

It is quite common for feeds (e.g. Facebook as reported above) to have
some MAYBE_HTML section HTML-encoded, which is currently not handled by
SimplePie
```xml
<title><![CDATA[ L&#039;alpha 11 est arriv&#xe9;e...]]></title>
```

This is the approach currently used (with success) in FreshRSS:
https://github.com/FreshRSS/FreshRSS/issues/754
https://github.com/FreshRSS/FreshRSS/pull/813

Alkarex/simplepie,Alkarex/simplepie,skyzyx/simplepie,simplepie/simplepie,mblaney/simplepie,FreshRSS/simplepie,Alkarex/simplepie,FreshRSS/simplepie,mblaney/simplepie,simplepie/simplepie,skyzyx/simplepie