Fix pure Python version mutating existing MultiDict when creating along with kwargs (#1113)

Noticed there was no copy here and the test shows we were mutating the
original.

Author: bdraco

CHANGES/1113.bugfix.rst

@@ -0,0 +1 @@
+Fixed the pure Python version mutating the original :class:`multidict.MultiDict` when creating a new :class:`multidict.CIMultiDict` from an existing one when keyword arguments are also passed -- by :user:`bdraco`.

multidict/_multidict_py.py

@@ -630,6 +630,7 @@ def _extend(
             if isinstance(arg, (MultiDict, MultiDictProxy)):
                 items = arg._impl._items
                 if kwargs:
+                    items = items.copy()
                     for key, value in kwargs.items():
                         items.append((self._title(key), key, value))
             else:

tests/test_multidict.py

@@ -1189,3 +1189,11 @@ def test_items_case_insensitive_isdisjoint(
     ) -> None:
         d = cls([("KEY", "one")])
         assert d.items().isdisjoint(arg) == expected
+
+
+def test_create_multidict_from_existing_multidict_new_pairs() -> None:
+    """Test creating a MultiDict from an existing one does not mutate the original."""
+    original = MultiDict([("h1", "header1"), ("h2", "header2"), ("h3", "header3")])
+    new = MultiDict(original, h4="header4")
+    assert "h4" in new
+    assert "h4" not in original