auto_schema with ServiceX processor does not work for non-jagged data with underlines in names

[alexander-held]

Describe the bug
The automatic schema used with ServiceX groups together objects like jet_e, jet_pt etc., and that works fine as long as they have a jagged structure. Non-jagged variables, like met_e, break the schema.

To Reproduce
Feed a flat column to a processor via ServiceX and give the column a name involving _.

Expected behavior
met_e, met_pt and similar variables get grouped into a met object.

Output
n/a

Desktop (please complete the following information):
coffea 0.7.14

Additional context
n/a

[alexander-held]

Sorry for the rather brief description, I can get back to this in a few days with a full reproducer in case the setup is not clear.