Add support for UTF-8 BOM in SimplePie feed_or_html()

https://github.com/FreshRSS/FreshRSS/issues/1336

lib/SimplePie/SimplePie/Content/Type/Sniffer.php

@@ -256,7 +256,12 @@ class SimplePie_Content_Type_Sniffer
 	public function feed_or_html()
 	{
 		$len = strlen($this->file->body);
-		$pos = strspn($this->file->body, "\x09\x0A\x0D\x20");
+		$pos = 0;
+		if (isset($this->file->body[2]) && $this->file->body[0] === "\xEF" &&
+			$this->file->body[1] === "\xBB" && $this->file->body[2] === "\xBF") {
+			$pos += 3;	//UTF-8 BOM
+		}
+		$pos += strspn($this->file->body, "\x09\x0A\x0D\x20", $pos);
 
 		while ($pos < $len)
 		{