add meaningful name to the example server

this server name will appear at autobahn summary report.
if we do not set the server name, it will display ugly `unknown server`.

autobahn/fuzzingclient.json

@@ -2,6 +2,7 @@
     "outdir": "./reports/servers",
     "servers": [
         {
+            "name": "nim-ws example server",
             "url": "ws://127.0.0.1:8888/ws"
         }
     ],

examples/server.nim

@@ -27,6 +27,11 @@ proc process(r: RequestFence): Future[HttpResponseRef] {.async.} =
 
       except WebSocketError as exc:
         error "WebSocket error:", exception = exc.msg
+
+    let header = HttpTable.init([
+      ("Server", "nim-ws example server")
+    ])
+
     discard await request.respond(Http200, "Hello World")
   else:
     return dumbResponse()