Create tree field with pk in dims resource

This fixes the case where calls to cached methods would fail during cache key construction because the pk was missing.

Signed-off-by: Don Naegely <naegelyd@gmail.com>

serrano/resources/field/dims.py

@@ -36,7 +36,8 @@ def get(self, request, pk):
 
         tree = trees[params.get('tree')]
         opts = tree.root_model._meta
-        tree_field = DataField(app_name=opts.app_label,
+        tree_field = DataField(pk='{}:{}'.format(tree, pk),
+                               app_name=opts.app_label,
                                model_name=opts.module_name,
                                field_name=opts.pk.name)