Why does Encode return an error?

```
The signature is:

func Encode(dst, src []byte) ([]byte, error)

The current implementation only ever returns nil for the errors.

Furthermore, how could this function ever return an error? Aren't any possible
values of dst and src valid?

For comparison, encoding/hex has

func Encode(dst, src []byte) int
func Decode(dst, src []byte) (int, error)
```

Original issue reported on code.google.com by `cespare@gmail.com` on 1 Oct 2014 at 11:22