Add quotes when marshaling a json.Number with the string option
set via a struct tag. This ensures that the resulting json
can be unmarshaled into the source struct without error.

Fixes #34268

Change-Id: Ide167d9dec77019554870b5957b37dc258119d81
GitHub-Last-Rev: dde81b71208be01c253bb87dbb6f81ac6e0785be
GitHub-Pull-Request: golang/go#34269
Reviewed-on: https://go-review.googlesource.com/c/go/+/195043


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>