fix dict merges with atomic base

bundlewrap/utils/dicts.py

@@ -216,7 +216,11 @@ def merge_dict(base, update):
     merged = base.copy()
 
     for key, value in update.items():
-        merge = key in base and not isinstance(value, _Atomic)
+        merge = (
+            key in base and
+            not isinstance(value, _Atomic) and
+            not isinstance(base[key], _Atomic)
+        )
         if merge and isinstance(base[key], dict):
             merged[key] = merge_dict(base[key], value)
         elif (

tests/unit/metadata.py

@@ -2,7 +2,21 @@
 from __future__ import unicode_literals
 
 from bundlewrap.utils.dicts import merge_dict
-from bundlewrap.metadata import blame_changed_paths
+from bundlewrap.metadata import atomic, blame_changed_paths
+
+
+def test_atomic_no_merge_base():
+    assert merge_dict(
+        {1: atomic([5])},
+        {1: [6, 7]},
+    ) == {1: [6, 7]}
+
+
+def test_atomic_no_merge_update():
+    assert merge_dict(
+        {1: [5]},
+        {1: atomic([6, 7])},
+    ) == {1: [6, 7]}
 
 
 def test_blame_and_merge():