Use proper method to access a URL.

There I need the whole part of the url which goes after host:port, but path() method returns that part without query arguments.
Fixed race.

Methods of response_stream may be called from service manager and from thevoid, so it should be thread-safe.
Do not access a null pointer.

Method response_stream::on_error() may be called after response_stream::close(), so it should not try to use response_stream::m_request deleted by close().