fix(graphql): send connection_init message during handshake

The implementation of `graphql-transport-ws` fails on the first message as it's only sent if the connection controller is in the `connected` state. For this protocol, the state is instead set to `handshake` during the first `_write()`.

https://github.com/zino-hofmann/graphql-flutter/blob/02be959597f0ffe0c21a227a2d1fdb02b3f56831/packages/graphql/lib/src/links/websocket_link/websocket_client.dart#L250-L256

Without this, the connection fails with the following log:
```
flutter: Initialising connection
flutter: There was an error causing connection lost: Bad state: No element
flutter: Disconnected from websocket.
```

Author: othorin

packages/graphql/lib/src/links/websocket_link/websocket_client.dart

@@ -370,13 +370,18 @@ class SocketClient {
   }
 
   void _write(final GraphQLSocketMessage message) {
-    if (_connectionStateController.value == SocketConnectionState.connected) {
-      socketChannel!.sink.add(
-        json.encode(
-          message,
-          toEncodable: (dynamic m) => m.toJson(),
-        ),
-      );
+    switch (_connectionStateController.value) {
+      case SocketConnectionState.connected:
+      case SocketConnectionState.handshake:
+        socketChannel!.sink.add(
+          json.encode(
+            message,
+            toEncodable: (dynamic m) => m.toJson(),
+          ),
+        );
+        break;
+      default:
+        break;
     }
   }