Fix a reflection warning on a static method argument type
Fix a reflection warning on a static method argument type

eastwood said: "Reflection warning - call to static method
encodeHexString on org.apache.commons.codec.binary.Hex can't be
resolved (argument types: unknown)."

The `encodeHexString` method has several variants for different
argument types. Looking through the codebase it doesn't seem we
use the `hexify` method for anything but the bytes, so it's safe
to add a type hint here and get rid of the warning.

lvh/caesium