Force close only the read end of connections.

Previously, when a request's body data exceeded the expected length,
Rocket would shutdown both ends of the corresponding connection. This PR
changes the behavior so that only the read end of the connection, on
Rocket's side, is shutdown. This allows a response to be sent in the
future while still preventing DoS attacks due to exuberant data.

Fixes #386.

core/lib/src/data/data_stream.rs

@@ -40,7 +40,7 @@ pub fn kill_stream(stream: &mut BodyReader) {
         Ok(FLUSH_LEN) | Err(_) => {
             warn_!("Data left unread. Force closing network stream.");
             let (_, network) = stream.get_mut().get_mut();
-            if let Err(e) = network.close(Shutdown::Both) {
+            if let Err(e) = network.close(Shutdown::Read) {
                 error_!("Failed to close network stream: {:?}", e);
             }
         }