Add doc_id handling to Table.Upsert() (#394)

[benetherington]

Allows upsert to accept a Document with a doc_id attribute.

The cond argument is now optional if a Document (with doc_id) is passed.

[msiemens]

Thanks for your work! I've thought about this for a bit and I think this is the best solution of adding doc_id support to upsert considering the differences of doc_id handling between update and insert.

The only thing that's left in my eyes is adding documentation about this in the usage documentation, but I think for now I'll just add a note about this in #371 so I'll remember this when reworking the documentation.

[msiemens]

I'll try to get a release published in the next 1/2 weeks 🙂

[benetherington]

I'd love to help with documentation, but it's so readable that I don't want to touch it. Thank you for your hard work!