PoC Add JSONBoundField to serializers (Fixes #4999)

Per issue #4999, JSONFields are not rendered properly in the DRF
browsable API HTML forms.  This patch attempts to fix that behavior by
introducing a JSONBoundField helper similar to the NestedBoundField
helper.

rest_framework/serializers.py

@@ -38,7 +38,8 @@ from rest_framework.utils.field_mapping import (
     get_relation_kwargs, get_url_kwargs
 )
 from rest_framework.utils.serializer_helpers import (
-    BindingDict, BoundField, NestedBoundField, ReturnDict, ReturnList
+    BindingDict, BoundField, JSONBoundField, NestedBoundField, ReturnDict,
+    ReturnList
 )
 from rest_framework.validators import (
     UniqueForDateValidator, UniqueForMonthValidator, UniqueForYearValidator,
@@ -521,6 +522,8 @@ class Serializer(BaseSerializer):
         error = self.errors.get(key) if hasattr(self, '_errors') else None
         if isinstance(field, Serializer):
             return NestedBoundField(field, value, error)
+        if isinstance(field, JSONField):
+            return JSONBoundField(field, value, error)
         return BoundField(field, value, error)
 
     # Include a backlink to the serializer class on return objects.

rest_framework/utils/serializer_helpers.py

 from __future__ import unicode_literals
 
 import collections
+import json
 from collections import OrderedDict
 
 from django.utils.encoding import force_text
@@ -82,6 +83,12 @@ class BoundField(object):
         return self.__class__(self._field, value, self.errors, self._prefix)
 
 
+class JSONBoundField(BoundField):
+    def as_form_field(self):
+        value = json.dumps(self.value)
+        return self.__class__(self._field, value, self.errors, self._prefix)
+
+
 class NestedBoundField(BoundField):
     """
     This `BoundField` additionally implements __iter__ and __getitem__