Cannot save or preview metadata after setting preserve metadata

[giohappy]

• upload shapefile + metadata --> ok
  • edit / edit metadata / tab settings --> check "metadata upload preserve"
  • "update"
  • edit / edit metadata / tab preview --> error

- 

• Response:

  {
  "success": false,
  "errors": "Note: this dataset's orginal metadata was populated and preserved by importing a metadata XML file. This metadata cannot be edited."
  }

NOTE: this happens at least since GeoNode 3.1.