TableRowJsonCoder looses type

When we create a TableRow (e.g. in a row function in bigQueryIO) using new TableRow().set(), for ex a long gets boxed into a Long. But when it is encoded using TableRowJsonCoder and then re-read it might be decoded as an Integer if the value fits into Integer. It causes failure in asserts in tests like write then read.

Link to the discussion in the ML:

https://lists.apache.org/thread.html/727fd63b2aa4890bed5953574a7e01485ef785de4558fd2327136ab8@%3Cdev.beam.apache.org%3E

Imported from Jira [BEAM-4734](https://issues.apache.org/jira/browse/BEAM-4734). Original Jira may contain additional context.
Reported by: echauchot.