Fix datafind trend discovery
This PR fixes an apparently critical problem with gwpy.io.datafind.find_frametype
that may have been introduced by #1376 where trend requests weren't properly parsed, so the wrong trend type may have been selected.
This was flagged by the test_find_best_frametype_ligo
test function, but that only runs during the cron nightly build at Caltech, which isn't part of the github CI.