BTC Markets: Fix sending trades to the websocket DataHandler (#1847)

* Fix websocket trade streaming

* Fix lint

* Rename trade.Data var

* Update wsTrade side type

* Add test for the invalid side

* Test the side

testdata/configtest.json

@@ -297,7 +297,9 @@
     },
     "enabled": {
      "autoPairUpdates": true,
-     "websocketAPI": false
+     "websocketAPI": false,
+     "saveTradeData": false,
+     "tradeFeed": true
     }
    },
    "bankAccounts": [