remodel.writePlaceFile and remodel.writeModelFile do not write Part0 and Part1 properties about remodel

[fredgig]

This issue now makes Remodel partly obsolete for us. It is no longer possible for us to publish our games with remodel.writeExistingPlaceAsset because WeldConstraint's are messed up in the process.

[NobleDraconian]

This issue is still happening. I've attached a repro project file below.

Rojo7_RefBug_Repro.zip

Steps to reproduce
From the root of the repro project, run these commands:

1. Run rojo build default.project.json -o Temp\CurrentBuild.rbxl

2. Open the generated place file. Observe that Workspace.WeldedModel.A.WeldConstraint has its ref values (part0 and part1) intact.
   

3. Close the place file, and run remodel run AddGameVersion.lua.

4. Open the place file. Observe that Workspace.WeldedModel.A.WeldConstraint has had its ref values wiped.
   

[badcc]

This is potentially same issue as rojo-rbx/rojo#458, I just encountered on =0.9.1