Add the management of incoming connection

The framework provides now a HTTP proxy, that manages
the incoming connections.
In other terms, the Connection headers are now removed,
and the connection is closed after that the response
has been sent back.

- HTTPHeaders: HTTPHeaders#delete has been added.

- HTTPIncomingConnection: HTTPIncomingConnection#get has been
  modified in order to take a block as input, that makes
  the expected changes on the incoming requests.

- HTTPProxy: HTTPProxy#start has been modified in order to
  manage the state of the incoming connection.