find_by_headers returned wrong table when using nested tables

[sintemal]

find_by_headers would stop to early when scanning for headers, as it would treat headers of nested tables as headers of the top table

[sintemal]

ping

[mk12]

Sorry, I'm not sure why I never saw this. Mentioning me (@mk12) might be more reliable.

I'm willing to merge this fix with two changes:

• It should be possible to do it directly with CSS selectors. I think :scope > th instead of th would be sufficient. If that doesn't work, there's probably some way to refactor it to use tr > th.
• It needs a test.