more flexible class selection
radiobuttons_convert_element only works if the radio element
being replaced has only the class "content" assigned to it, because that
can enable other radio elements (such as pricesets, which have
"contribution_amount-content" specified as an additional class). Without
this change, the extension can still replace the radio element with
buttons but the feature that indicates that an option is selected (and
changes the background color) never matches.