Filter by author, sort both first and secondary authors

Since I updated to this new version, the filter by author sort nodes by both first and secondary author. Let's say that the first author is Karl Marx and the secondary author is Friedrich Engels, then the node will be sorted twice, first under E (but in fact the header will show M) and then under M (with the right header M).
I have updated from 6x-1.0-rc5.

Thanks for the great module!