If all credit types are included in a search (hard coded at a count of 6 right now because I don't want to take the effort required to track whether or not "all" are still checked and send that back and forth) then we disregard the criteria searching on credit types altogther.

At the same time, we're looking for >=0 credits, so that will allow 0 credit and fractional credits, in the case that < 6 credit types are specified in a search.

This and the UI change should be all that's needed to close OSCPA/Store-Pathfix#163.