[BUG] invalid arguments passed into fileserver.update from saltutil runner

Description
Running the fileserver.update through saltutil.runner causes extra argument __pub_user to be passed into the function

Setup
Salt 3006.x and above.

Steps to Reproduce the behavior
Run: salt master_minion saltutil.runner fileserver.update

Error:

    Passed invalid arguments: update() got an unexpected keyword argument '__pub_user'

    Usage:

        Update the fileserver cache. If no backend is provided, then the cache for
        all configured backends will be updated.

        backend
            Narrow fileserver backends to a subset of the enabled ones.

            .. versionchanged:: 2015.5.0
                If all passed backends start with a minus sign (``-``), then these
                backends will be excluded from the enabled backends. However, if
                there is a mix of backends with and without a minus sign (ex:
                ``backend=-roots,git``) then the ones starting with a minus
                sign will be disregarded.

                Additionally, fileserver backends can now be passed as a
                comma-separated list. In earlier versions, they needed to be passed
                as a python list (ex: ``backend="['roots', 'git']"``)

        kwargs
            Pass additional arguments to backend. See example below

        CLI Example:

        .. code-block:: bash

            salt-run fileserver.update
            salt-run fileserver.update backend=roots,git
            salt-run fileserver.update backend=git remotes=myrepo,yourrepo

Expected behavior

master_minion:
    True

Additional context
Was introduced with this commit: https://github.com/saltstack/salt/commit/d5b31df509c45f7d67e5b16900262400550a7ab5

Workaround:
Edit modules/saltutil.py & comment out:

pub_data["user"] = kwargs.pop("__pub_user", "UNKNOWN")

Versions Report

Start in modules/saltutil.py at the handling of __pub_user and trace the saltutil.runner path for fileserver.update. Reproduce with salt master_minion saltutil.runner fileserver.update and verify that the call no longer passes an invalid argument and returns the expected master_minion: True result.