reply's send_error() method fixed.

According to the documentation about the method in comment section
in stream.hpp send_error() is a shortcut for:
* ioremap::http_response response(type);
* send_headers(std::move(response), boost::asio::const_buffer(),
* std::bind(&reply_stream::close, reply, std::placeholders::_1));

Before it called send_headers() method with close_impl() callback and
close() that led to another close_impl() call.
connection: clear m_content_length on next request.
connection: terminate connection on invalid url and if appropriate handler is not found.
connection: unnecessary code removed.