OBI:0100026 as taxon for variant objects

[deepakunni3]

This bug was originally raised by @iimpulse

For reference, the query: https://api-dev.monarchinitiative.org/api/bioentity/gene/MGI:98297/variants?fetch_objects=true&start=0&rows=10&facet=true&facet_fields=subject_taxon&taxon=OBI%3A0100026

yields gene to variant associations. But there are some associations where object has a BNODE prefix, and a taxon of OBI:0100026 'organism'. This makes it difficult to filter variants based on taxon since 'organism' is too generic of a taxon term.

@cmungall @kshefchek Thoughts?

CC'ing @monicacecilia for her awesomeness!