fix galah_group_by() for multiple + single taxa

[due23]

Haven't tested any other cases to see whether they differ from one another between python and R but this one did so suggests the count isn't quite working for the group by.

Python version:
taxa_array = ["Swainsona formosa", "Crocodylus johnstoni", "Platalea (Platalea) regia", "Notamacropus agilis"]
f = ["dataResourceName = iNaturalist Australia", "year=2022"]
group_by = ["month"]
galah.atlas_counts(taxa_array, filters=f, group_by=group_by, expand=False)

R version:

Also found it's not quite working the way it should for some single taxa either: galah.atlas_counts(taxa = "Centrostephanus rodgersii", group_by="year", expand=False) returns below when there are definitely records in the 2000s

Python version

R version: