Fixed `swap` between dict items.

benedict/core/swap.py

@@ -4,4 +4,8 @@
 def swap(d, key1, key2):
     if key1 == key2:
         return
-    d[key1], d[key2] = d[key2], d[key1]
+    val1 = d[key1]
+    val1 = val1.copy() if isinstance(val1, dict) else val1
+    val2 = d[key2]
+    val2 = val2.copy() if isinstance(val2, dict) else val2
+    d[key1], d[key2] = val2, val1

tests/dicts/test_benedict.py

@@ -2081,6 +2081,22 @@ b:
         b = benedict(d)
         b.swap("a.y", "b.y")
         b.swap("b.x", "c.x")
+        r = {
+            "a": {
+                "x": 1,
+                "y": 2,
+            },
+            "b": {
+                "x": 3,
+                "y": 1,
+            },
+            "c": {
+                "x": 2,
+                "y": 3,
+            },
+        }
+        self.assertEqual(b, r)
+
         b.swap("a", "c")
         r = {
             "a": {