mutate_in doesn't work as expected in couchbase==3.0.0b3.dev28+g2137539d

On build 7.0.0 build 1538

Steps:

1. create a sdk client insert a doc 
2. mutate_in that doc

self.client.cb.default_collection().mutate_in(
KEY, SD.upsert('_system1', val, xattr=True, create_parents=True))

mutate_in fails with below error

  File "pytests/subdoc/subdoc_simple_dataset.py", line 35, in test_system_xattr_with_compression

    KEY, SD.upsert('_system1', val, xattr=True, create_parents=True))

  File "/Users/anithakuberan/Documents/testlib/venv/lib/python3.7/site-packages/couchbase/collection.py", line 820, in mutate_in

    return MutateInResult(self.bucket.mutate_in(key, spec, **final_options), **final_options)

  File "/Users/anithakuberan/Documents/testlib/venv/lib/python3.7/site-packages/couchbase_core/client.py", line 409, in mutate_in

    return super(Client, self).mutate_in(key, tuple(specs), **kwargs)

couchbase_core.exceptions.ArgumentError: <Expected tuple for spec, C Source=(src/oputil.c,819), OBJ=5>

Please let me know in case any other log is required