Minor changes necessary to run `go test` under 1.15

[cite-reader]

In Go 1.15, converting from non-byte or -rune integer types directly to a string became a go vet warning, enabled by default when running go test. Goutils was using this conversion correctly, so mechanically wrapping an additional conversion to rune (as suggested by the Go 1.15 changelog) at the warning sites allows go test to run without errors on recent Go toolchains.