Fix headers not cleared between multiple requests on same connection (HT...

Hi, 

The HTTPSession object will handle multiple requests on the same connection by calling execute in a loop. But it looks like the http headers are not cleaned up between requests. 

For example; it a GET follows a POST (eg. favicon.ico) the headers will still contain an incorrect header-length, leading to a socket timeout as the server will be waiting for non existent data.

I think this simple fix will solve the problem. Unless there is a reason to keep the headers?

Cheers, Rien
@philippwiesemann How about removing all headers except them that can't change (like "remote-addr" and "http-client-ip")?

Because I also had the problem that the "content-length" field stays the same if not re-sent by the client which makes NanoHTTPD think there are more body bytes to read even though there are none. This results in lots of read timeouts which really spam the log.