fix: prevent button style from being modified in before_update()

[ndonkoHenri]

Test Code

import flet as ft

def main(page: ft.Page):

    page.add(
        button := ft.ElevatedButton(
            "Button",
            style=ft.ButtonStyle(
                bgcolor=ft.colors.GREEN,
                color=ft.colors.RED,
                elevation=10,
                shape=ft.RoundedRectangleBorder(4),
                text_style=ft.TextStyle(
                    color=ft.colors.BLUE,
                ),
            ),
        )
    )
    print(button.style.shape)
    print(button.style.text_style)


ft.app(main)