Fix and refactoring of some procedures which are able to return nil as result (#97)

* Fix do not return nil as result.

* Fix mplex test to properly raise.

libp2p/protocols/secure/secure.nim

@@ -20,7 +20,9 @@ import secureconn,
 type
   Secure* = ref object of LPProtocol # base type for secure managers
 
-method handshake(s: Secure, conn: Connection, initiator: bool = true): Future[SecureConn] {.async, base.} =
+method handshake(s: Secure,
+                 conn: Connection,
+                 initiator: bool = true): Future[SecureConn] {.async, base.} =
   doAssert(false, "Not implemented!")
 
 proc readLoop(sconn: SecureConn, stream: BufferStream) {.async.} =