proto: convert integer to rune before converting to string

[saser]

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.

Fixes #1209.

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[dsnet]

We consider development of the github.com/golang/protobuf module frozen at this point. However, it is an important goal that the module continues to work with higher versions of Go. Therefore, this change is justified.

The PR looks good to me, but will require the CLA to be signed in order to be merged.

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[googlebot]

CLAs look good, thanks!

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[saser]

Thank you for merging and for the very quick response time! :)