fix decoding of table rows with variant types

[fschoell]

Background

Decoding of table rows fails if the table type is a variant. Example error from dfuse:

row struct "variant<account_v0>", err: structure [variant<account_v0>] not found in abi

Summary

fixed this by having decode() check first if the given struct name is a variant and if so use the variant type instead.

Note

Checklist

• [x] Backward compatible?
• [ ] Test enough in your local environment?
• [x] Add related test cases?