Switch between multipart alternatives

[Drew Raines <aaraines@gmail.com>]

A lot of email nowadays, especially through Google Groups, is
multipart/alternative.  For a while now I've had
gnus-inhibit-mime-unbuttonizing set so that I get these at the top of
an article:

      1.  ( ) text/plain          (*) text/html           

...which, eg if I don't like the rendering of the html, lets me know
that I have a text/plain option (although many times the html through
shr.el looks really good!).

I used to be able to do this with `b' which would quickly switch
between these parts, but that doesn't work anymore.  I know I can
just hit the buttons and toggle that way but it takes too long to
navigate over there.  I liked the simplicity and speed of the former
method.

I've noticed it has been this way for months, well before Ma Gnus.
I've just been lazy figuring out what happened.  Anybody know what I
can frob to get this behavior back?  Or is there a better way to
accomplish what I was doing?

-Drew