Instead of using DjangoJSONEncoder, use base_field's value_to_string().

Note this means the serialization of e.g. IntegerRangeField now has
strings for lower and upper, so use to_python when they came back in
(same behaviour as ArrayField, hopefully, from where I also got the
set_attributes_from_name function).

Backport of 86d9b10d and
8a842148 from master