proto: convert integer to rune before converting to string (#1210)
proto: convert integer to rune before converting to string (#1210)

Go 1.15 introduced a new `go vet` warning
(https://golang.org/doc/go1.15#vet) for conversions of the form
`string(x)` where `x` is an integer type other than `rune` or `byte`.
This warning is enabled by default when running `go test`. As a
consequence, running `go test github.com/golang/protobuf/proto`
results in a build failure prior to this commit.
golang/protobuf,golang/protobuf