[0.15.3] Rail signals may not attach on BP

[aaargha]

This is probably this old bug that refuses to die (it could also happen in 0.14).

Basically when building blueprints there is a chance that the signals will not properly attach to the rails, they're left blinking and useless.

I've attached a reproduction save with some examples of the result. It seems to be more likely to happen if the rails are built first.

To use the reproduction save:
Replace the chest containing signals with a requester chest.
Place the blueprint that is in the tool belt.
After the rails are built replace the chest containing signals with a passive provider.
Repeat as needed.

The results are not consistent: some times one signal is broken, sometimes others, sometimes none. It might be related to the order in which they are placed?

[Rseding91]

Thanks for the report. This should be fixed for the next version of 0.15.