A default.project.json with an omitted name field sets the name of its top-level instance to default after subsequent changes

[kennethloeffler]

This issue is reproducible on latest mater, and is related to #858 .

Repro steps:

1. Create a default.project.json and omit the name field. I used the following project: no_name_top_level_project.zip.
2. Run rojo serve on the project, and visit http://localhost:34872/show-instances. Observe that the resulting instance has the correct name:
   
3. Make any change to the project file (I added a newline). Refresh http://localhost:34872/show-instances and observe that the instance's name has changed to default: