product_ref_group is nil when loading a project

When loading a project from file the product_ref_group is nil even if the file contains the Products group.
Initialize_from_scratch initializes the variable: https://github.com/CocoaPods/Xcodeproj/blob/master/lib/xcodeproj/project.rb#L184

However, initialize_from_file does not:
https://github.com/CocoaPods/Xcodeproj/blob/master/lib/xcodeproj/project.rb#L198

I would like to contribute a fix, however I am not sure where i can best add a test to reproduce the issue.