Unify JSONBoundField as_form_field output between py2 and py3

When using json.dumps with indenting, in python2 the default formatting
prints whitespace after commas (,) and python3 does not. This can be
unified with the separators keyword argument.

rest_framework/utils/serializer_helpers.py

@@ -90,7 +90,12 @@ class JSONBoundField(BoundField):
         # value will be a JSONString, rather than a JSON primitive.
         if not getattr(value, 'is_json_string', False):
             try:
-                value = json.dumps(self.value, sort_keys=True, indent=4)
+                value = json.dumps(
+                    self.value,
+                    sort_keys=True,
+                    indent=4,
+                    separators=(',', ': '),
+                )
             except (TypeError, ValueError):
                 pass
         return self.__class__(self._field, value, self.errors, self._prefix)