Logic for serializing the default value in textual form exists in
multiple places in trivially similar forms. Centralize that logic.
Change-Id: I4408ddfeef2c0dfa5c7468e01a4d4df5654ae57f
Reviewed-on: https://go-review.googlesource.com/c/153022
Reviewed-by: Herbie Ong <herbie@google.com>
