Trivial Custom cable coils always use synthrubber

[MrMagolor]

I tried making a fibrilith-insulated gnesis cable coil, for the purposes of laser guns. When the cable was produced, though, it was listed as a "synthrubber-insulated gnesis cable coil", and even the description said it was insulated with synthrubber.

Minor bug since the conductor is what matters, but still this is a weird bug.