The [[plugins/img]] plugin is not generating the proper `class`
attribute in its HTML output.

The plugin receives something like the following:

    \[[!img 129199047595759991.jpg class="centered"]]

And is supossed to generate an HTML code like the following:

    <img src="129199047595759991.jpg" class="centered" />

But is generating the following

    <img src="129199047595759991.jpg" class="centered img" />

This seems to be happening with all images inserted using the plugin (that use
the `class=yaddayadda` argument to the `img` directive.) I remember it didn't
happen before, and I suspect an ikiwiki upgrade is to blame. I tested with a
blog created from scratch, and a single post, and the problem appeared there
too.

This is happening with version 3.20100815 of ikiwiki.

[[jerojasro]]