Attach ws event listeners using EventTarget API

When using `.on<event>=fn` to attach listeners, only one listener can be
set at the same time. Since multiple request managers can use the same
provider, the EventTarget API has to be used to ensure all of them
receive the events emitted from the provider.

This is needed on both the `on` and `removeListener` functions.

packages/web3-providers-ws/src/index.js

@@ -305,15 +305,15 @@ WebsocketProvider.prototype.on = function (type, callback) {
             break;
 
         case 'connect':
-            this.connection.onopen = callback;
+            this.connection.addEventListener('open', callback);
             break;
 
         case 'end':
-            this.connection.onclose = callback;
+            this.connection.addEventListener('close', callback);
             break;
 
         case 'error':
-            this.connection.onerror = callback;
+            this.connection.addEventListener('error', callback);
             break;
 
         // default:
@@ -342,7 +342,17 @@ WebsocketProvider.prototype.removeListener = function (type, callback) {
             });
             break;
 
-        // TODO remvoving connect missing
+        case 'connect':
+            this.connection.removeEventListener('open', callback);
+            break;
+
+        case 'end':
+            this.connection.removeEventListener('close', callback);
+            break;
+
+        case 'error':
+            this.connection.removeEventListener('error', callback);
+            break;
 
         // default:
         //     this.connection.removeListener(type, callback);