Ensures that get_codec doesn't modify its argument.

Closes zarr-developers#78.

Author: jeromekelleher

numcodecs/registry.py

@@ -29,6 +29,7 @@ def get_codec(config):
     Zlib(level=1)
 
     """
+    config = dict(config)
     codec_id = config.pop('id', None)
     cls = codec_registry.get(codec_id, None)
     if cls is None:

numcodecs/tests/test_registry.py

@@ -11,3 +11,11 @@
 def test_registry_errors():
     with pytest.raises(ValueError):
         get_codec({'id': 'foo'})
+
+
+def test_get_codec_argument():
+    # Check that get_codec doesn't modify its argument.
+    arg = {"id": "json"}
+    before = dict(arg)
+    get_codec(arg)
+    assert before == arg