base_url getting appended twice in redirects when applying filters?

[EternityForest]

When running under a custom base URL, the "Apply" button for filters does not seem to work. It redirects to a url starting with /baseurl/baseurl/ and the extra copy of the base URL messes things up.

I'm able to get things working with the following changes, but I'm not sure if the whole thing is just user error on my end.
If not, I can make a PR.

class Urls:
    def __init__(self, ds):
        self.ds = ds

    def path(self, path, format=None, already_prefixed=False):
        if not isinstance(path, PrefixedUrlString):
            if path.startswith("/"):
                path = path[1:]
            if not already_prefixed:
                path = self.ds.setting("base_url") + path
            else:
                path = "/" + path
        if format is not None:
            path = path_with_format(path=path, format=format)
        return PrefixedUrlString(path)

async def _redirect_if_needed(datasette, request, resolved):
    # Handle ?_filter_column
    redirect_params = filters_should_redirect(request.args)
    if redirect_params:
        return _redirect(
            datasette,
            request,
            datasette.urls.path(path_with_added_args(request, redirect_params), already_prefixed=True),
            forward_querystring=False,
        )