Fix async pubsub

[roniemartinez]

Pubsub examples are failing when converted to async (pre-release).

For example, converting the code from https://flet.dev/docs/controls/page#send_allmessage to async

from dataclasses import dataclass
import flet as ft

@dataclass
class Message:
    user: str
    text: str


async def main(page: ft.Page):

    async def on_broadcast_message(message):
        await page.add_async(ft.Text(f"{message.user}: {message.text}"))

    await page.pubsub.subscribe_async(on_broadcast_message)

    async def on_send_click(e):
        await page.pubsub.send_all_async(Message("John", "Hello, all!"))

    await page.add_async(ft.ElevatedButton(text="Send message", on_click=on_send_click))


ft.app(target=main)

The above will throw the following error as soon as the button is clicked.

Traceback (most recent call last):
  File "/Users/roniemartinez/Library/Caches/pypoetry/virtualenvs/flet-test-ytD6Reby-py3.11/lib/python3.11/site-packages/flet/flet.py", line 345, in on_event
    await conn.sessions[e.sessionID].on_event_async(
  File "/Users/roniemartinez/Library/Caches/pypoetry/virtualenvs/flet-test-ytD6Reby-py3.11/lib/python3.11/site-packages/flet_core/page.py", line 459, in on_event_async
    await handler(ce)
  File "/Users/roniemartinez/Projects/flet-test/example.py", line 18, in on_send_click
    await page.pubsub.send_all_async(Message("John", "Hello, all!"))
  File "/Users/roniemartinez/Library/Caches/pypoetry/virtualenvs/flet-test-ytD6Reby-py3.11/lib/python3.11/site-packages/flet/pubsub.py", line 189, in send_all_async
    await self.__pubsub.send_all_async(message)
  File "/Users/roniemartinez/Library/Caches/pypoetry/virtualenvs/flet-test-ytD6Reby-py3.11/lib/python3.11/site-packages/flet/pubsub.py", line 31, in send_all_async
    await self.__send_async(handler, [message])
  File "/Users/roniemartinez/Library/Caches/pypoetry/virtualenvs/flet-test-ytD6Reby-py3.11/lib/python3.11/site-packages/flet/pubsub.py", line 177, in __send_async
    asyncio.create_task(handler(**args))
                        ^^^^^^^^^^^^^^^
TypeError: __main__.main.<locals>.on_broadcast_message() argument after ** must be a mapping, not list

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[roniemartinez]

Related to my comment in #128 (comment)

[FeodorFitsner]

Thank you! I'm going to merge it as soon as PR build is finished.