dprint_r() doesn't html encode the contents of the variables it's displaying

dprint_r() prints the contents of the variables directly to screen; but if the contents include html tags, you won't be able to see the contents and it can mess up your template's html.

This same type of problem looks to have been fixed in devel_show_object(), but not in dprint_r().

This patch applies check_plain() before printing. It also display the variable like this: 'name => value' instead of this: '   name value'