Authors/contributors/people are lost from forms if a validation error occurs

[fbacall]

To reproduce:

• Add some authors/contributors to a material
• Make the title blank
• Try and save the material

The authors/contributors still appear to be set in the form, but the names of each input is e.g. event[contributors][replace-me][name]

and upon fixing the validation errors and saving the material, the authors/contributors will not be saved