Set copy flags when copying container.

When copying a DX container which has AT children, the UID of the AT
children was not updated.
The reason for the error is that the DX container copy did not have the
_v_is_cp flag while the AT children were processed and thus the flag was
not properly delegated.

By copying the _v_is_cp and _v_cp_refs flags to the copy we have the
same behavior as it used to be with AT, which does fix the error.

CHANGES.rst

@@ -14,7 +14,8 @@ New features:
 
 Bug fixes:
 
-- *add item here*
+- Fix error when copying DX containers with AT children which caused the
+  children to not have the UID updated properly.  [jone]
 
 
 2.2.7 (2016-05-05)

plone/dexterity/content.py

@@ -221,6 +221,22 @@ def _verifyObjectPaste(self, obj, validate_src=True):
                         'You can not add the copied content here.'
                     )
 
+    def _getCopy(self, container):
+        # Copy the _v_is_cp and _v_cp_refs flags from the original
+        # object (self) to the new copy.
+        # This has impact on how children will be handled.
+        # When the flags are missing, an Archetypes child object will not have
+        # the UID updated in some situations.
+        # Copied from Products.Archetypes.Referenceable.Referenceable._getCopy
+        is_cp_flag = getattr(self, '_v_is_cp', None)
+        cp_refs_flag = getattr(self, '_v_cp_refs', None)
+        ob = super(PasteBehaviourMixin, self)._getCopy(container)
+        if is_cp_flag:
+            setattr(ob, '_v_is_cp', is_cp_flag)
+        if cp_refs_flag:
+            setattr(ob, '_v_cp_refs', cp_refs_flag)
+        return ob
+
 
 @implementer(
     IDexterityContent,