[BUG] set_props called multiple times only keep the last props.

[T4rk1n]

For regular callbacks, when multiple call of set_props to the same component id, only the last call is saved.

Example:

from dash import Dash, Input, html, set_props

app = Dash()

app.layout = [
    html.Button("start", id="start"),
    html.Div("initial", id="output"),
]

@app.callback(
    Input("start", "n_clicks"),
)
def on_click(_):
    set_props("output", {"children": "changed"})
    set_props("output", {"style": {"background": "red"}})

if __name__ == "__main__":
    app.run(debug=True)

Clicking on the start button only set the background red, the text stays at "initial". The props should be merged and both updated.