some unicode attributes cannot be roundtripped

[dcherian]

key = '/0/zarr.json'
value = b'{\n  "shape": [\n    1\n  ],\n  "fill_value": false,\n  "chunk_grid": {\n    "name": "regular",\n    "configuration": {\n      "chunk_shape": [\n        1\n      ]\n    }\n  },\n  "attributes": {\n    "\\ud800": null\n  },\n  "zarr_format": 3,\n  "data_type": "bool",\n  "chunk_key_encoding": {\n    "name": "default",\n    "configuration": {\n      "separator": "/"\n    }\n  },\n  "codecs": [\n    {\n      "name": "bytes",\n      "configuration": {\n        "endian": "little"\n      }\n    }\n  ],\n  "node_type": "array"\n}'
await self._store.set(key, value)

ValueError: store error: bad metadata: `unexpected end of hex escape at line 15 column 12`

seem like an edge case with handling "attributes": {\n "\\ud800": null\n } but perhaps there's a real issue here around serializing this kind of thing.

Seen in #68