fix: missing space for repeating unary operator

[CompuIves]

Whenever you have this code:

c = (10.0, + +(15))

astring will convert it to

c = (10.0, ++(15))

which is invalid. This change makes sure it prints correctly.

I'm not sure where I should specify a test for this, any particular place?

[davidbonnet]

Thanks a lot @CompuIves for reporting and fixing this bug.