Add default for Mat4:unpack()

[jmiskovic]

A small fix; Quat already has this.

[bjornbytes]

I can never decide if booleans that default to false should have an explicit default or if it should be left off since nil is falsy. It's good to be consistent though.

[jmiskovic]

I always preferred when docs were explicit and didn't require extra reasoning from my side. If the default is not mentioned I would feel the need to check the code, as wrong assumptions have bitten me enough times already.