fix params duplicate bug (#2421)

Author: DeNeutoy

allennlp/common/params.py

@@ -372,7 +372,7 @@ def duplicate(self) -> 'Params':
         Uses ``copy.deepcopy()`` to create a duplicate (but fully distinct)
         copy of these Params.
         """
-        return Params(copy.deepcopy(self.params))
+        return copy.deepcopy(self)
 
     def assert_empty(self, class_name: str):
         """

allennlp/tests/common/params_test.py

@@ -345,3 +345,14 @@ def test_infer_and_cast(self):
 
         assert params.as_dict() == lots_of_strings
         assert params.as_dict(infer_type_and_cast=True) == casted
+
+    def test_duplicate_copies_all_params_state(self):
+
+        params = Params({},
+                        loading_from_archive=True,
+                        files_to_archive={"hey": "this is a path"})
+
+        new_params = params.duplicate()
+
+        assert new_params.loading_from_archive
+        assert new_params.files_to_archive == {"hey": "this is a path"}