Reduce amount of work done when unpacking unprintable characters.

Instead of going through the fmt package, we can use append int,
which saves an allocation.

benchmark                                old ns/op     new ns/op     delta
BenchmarkUnpackDomainNameUnprintable     2147          506           -76.43%
Merge pull request #149 from DanielMorsing/unprintable

Reduce amount of work done when unpacking unprintable characters.