Unit tests do not run with go1.15 due to vet warnings

[gammazero]

When using go1.15, tests work when running with go test -vet=off, but when running go test or go vet the following appears:

# github.com/multiformats/go-multibase
./multibase.go:87:10: conversion from untyped int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
./multibase_test.go:27:21: conversion from untyped int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
./multibase_test.go:94:23: conversion from Encoding (int) to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
./multibase_test.go:99:22: conversion from Encoding (int) to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
./multibase_test.go:104:22: conversion from Encoding (int) to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
./multibase_test.go:109:22: conversion from Encoding (int) to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)