[release-branch.go1.2] encoding/gob: do not use MarshalText, UnmarshalText

««« CL 22770044 / 23fc3139589c
encoding/gob: do not use MarshalText, UnmarshalText

This seems to be the best of a long list of bad ways to fix this issue.

Fixes #6760.

R=r
CC=golang-dev
https://golang.org/cl/22770044
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/28110043