Changed column header interpretation for file upload.

This is similar to C0BRA's (haha) MaNtIcOre .rootrc <-> JSON conversion between
flat-hierarchy and JSON: columns a, b.b1, b.b2, b.c.d convert to the document
{ a: 12.3, b:{b1:"YES",b2:"NO",c:{d:"MAYBE"}} }.
I made the following improvements (which are unsupported by .rootrc but no
problem for xml/GeoJSON):
- Dots and spaces within property names may be written as
  agents.'of S.H.I.E.L.D'. This is a safety precaution to export and then
  re-import objects without losing structure.
- Properties can be deleted by uploading an empty string/null/"undefined" or
  whatever useless information might as well be conveyed by a missing property.
Removed redundant JSON components {}, [], null, "" etc.

doc.GeoJSON is excused because the many many points may cause too much
recursion.
Inserted a more meaningful 'err' in _list/tabulate.
I think prop() makes more sense here but I am still unsure.