auto_resume takes effect too early in command substitution

[Oğuz]

See:

    $ sleep 20
    ^Z
    [1]+  Stopped                 sleep 20
    $
    $ auto_resume= sleep
    sleep: missing operand
    Try 'sleep --help' for more information.
    $
    $ echo $(auto_resume= sleep)
    bash: fg: no current jobs

    $

`auto_resume= sleep' has the same effect as `auto_resume= command sleep' in
a
command substitution but I think it shouldn't; I'd expect a command to
behave
the same way both inside and outside `$(...)'.

This is reproducible on 5.1.0(14)-beta too.

Oğuz