HTTPClient makes trouble by gettinhg of RSS-feeds

Hi,

in my podcast app I try to collect RSS feeds. So far so good. A litte part of URls makes trouble (most are serendipity blogs): the server give me a HTML instead of XML. Normal browsers, wget and cURL works fine. But the Android HTTPclient has only this crazy behavior. After inspection of firefox console I send:

xhr.open('GET', _url, true);
xhr.setRequestHeader('User-Agent', 'Mozilla/5.0 (KHTML, like Gecko FeedBurn planet)');
xhr.setRequestHeader('Accept', 'application/rss+xml');
xhr.setRequestHeader('Cookie', null);
xhr.setRequestHeader('Accept-Encoding', 'gzip, deflate');
xhr.send();

Any ideas?