Fix rendering of field of type 'Document' in spreadsheet editor

Spreadsheet editor displays

[object Object]

when rendering a document property of type Document.

How to reproduce:

• in Studio Modeler define a document type named DocType1 (that extends nothing) and add to it a field named docref of type Document
• define its create layout to input field doctype1:docref
• define a content view named cv1:
  • set Query filter to ecm:mixinType != 'HiddenInNavigation' AND ecm:isVersion = 0 AND ecm:currentLifeCycleState != 'deleted' AND ecm:parentId = ?
  • add a query parameter with value

    #{currentDocument.id}

  • add a result column Document Id title with link
  • add a result column with a widget by property for field doctype1:docref
• define another document type named DocType2 that extends Folder:
  • check Show Create Child Action
  • set document type DocType1 as a child document type
  • set the Main content content view to cv1
• deploy and log in the Nuxeo platform
• create a folderish document of type DocType2
• create a document of type DocType1 under it and set its doctype1:docref to any other document
• navigate back to the listing of the folderish document of type DocType2
• click on the content view's Spreadsheet icon to open the spreadsheet editor

Actual result:

[object Object]

is rendered in the column for field doctype1:docref
Expected result:
Title and/or document path of document referenced in doctype1:docref

The bug was originally reported for the Web UI but I thought a reproduction scenario in the JSF UI would be simpler.