fix: handle non-object JSON messages in on_message_async

[AmSach]

Fixed the bug described in issue elliottech#123. Here's what was wrong and how I fixed it:

Problem: The on_message_async method crashes with AttributeError: 'str' object has no attribute 'get' when the WebSocket server sends a non-object JSON message (e.g., a string heartbeat). This is because json.loads('"heartbeat"') returns a Python str, and calling .get() on a string raises an AttributeError.

Root Cause: Unlike the sync on_message method, on_message_async skipped the isinstance check after parsing. It directly called message.get('type') assuming the parsed result was always a dict.

Fix: Added the same isinstance(message, dict) check that exists in on_message_sync. After parsing, if the result is not a dict, the method returns early — allowing the client to gracefully handle heartbeat messages without crashing.

Tested by: Verified the fix handles:

• Raw string heartbeat: returns early (non-dict)
• JSON string '"heartbeat"': parses to Python str, returns early
• Valid JSON object {'type': 'ping'}: processes correctly

Fixes: elliottech#123