add MultiDictView.copy

mitmproxy/types/multidict.py

@@ -230,3 +230,6 @@ class MultiDictView(_MultiDict):
     @fields.setter
     def fields(self, value):
         self._setter(value)
+
+    def copy(self):
+        return MultiDict(self.fields)

test/mitmproxy/types/test_multidict.py

@@ -205,3 +205,12 @@ class TestMultiDictView:
         tv["c"] = "b"
         assert p.vals == (("a", "b"), ("c", "b"))
         assert tv["a"] == "b"
+
+    def test_copy(self):
+        p = TParent()
+        tv = multidict.MultiDictView(p.getter, p.setter)
+        c = tv.copy()
+        assert isinstance(c, multidict.MultiDict)
+        assert tv.items() == c.items()
+        c["foo"] = "bar"
+        assert tv.items() != c.items()