Attributes are not updated when using a dictionary

[luator]

Minimal, reproducible code sample, a copy-pastable example if possible

Run the following twice:

import zarr

if __name__ == "__main__":
    foo = zarr.open_group("/tmp/foo.zarr")
    if "bla" in foo.attrs:
        print("after loading:", foo.attrs["bla"]["foo"])

    foo.attrs["bla"] = {"foo": 13}
    print("after initialize:", foo.attrs["bla"]["foo"])

    foo.attrs["bla"]["foo"] = 42
    print("after update:", foo.attrs["bla"]["foo"])

Problem description

If storing a dictionary in the attributes of a group, it will be initialised correctly but changes don't seem to be written to disk.

When running the above example multiple times, I would expect the following output:

after loading: 42
after initialize: 13
after update: 42

However, I am getting

after loading: 13
after initialize: 13
after update: 42

I do not have the issue when directly writing an integer to foo.attrs["bla"], only when using the dictionary.

Version and installation information

Please provide the following:

• Value of zarr.__version__: 2.6.1
• Value of numcodecs.__version__: 0.7.3
• Version of Python interpreter: 3.6.5
• Operating system: Ubuntu 18.04
• How Zarr was installed: pip3 install zarr