Pass the backend in holoviews.opts.defaults

[LecrisUT]

This is a quick issue I've found when trying to run:

holoviews.opts.defaults(
    holoviews.opts.Curve(
        fig_size=500,
        linewidth=1,
    ),
    backend="matplotlib",
)

Because the backend is not propagated when doing _expand_options you get:

WARNING:param.main: Option 'fig_size' for Curve type not valid for selected backend ('bokeh'). Option only applies to following backends: ['matplotlib']

It seems that the options are indeed not populated due to the warning (confirmed, that with this PR, the default is set appropriately)

[ahuang11]

Thank you for contributing! To make HoloViews more robust do you think you can add a test that checks if opts are propagated properly using hv.opts.defaults or alternatively checking if the warning is gone?

Maybe in https://github.com/holoviz/holoviews/blob/main/holoviews/tests/plotting/test_plotutils.py (not 100% certain).

[LecrisUT]

If I have a reference test, I could try and add that.