xcube Server issues with int16

[AliceBalfanz]

Describe the bug
A xarray data set with a variable in int16 cannot be visualized in xcube Viewer. The error from xcube Server is: "TypeError: Object of type int16 is not JSON serializable"

The same dataset loaded as float64 can be visualized via xcube Viewer.

To Reproduce
Steps to reproduce the behavior:

1. Create a dataset with a variable in int16
2. Serve with xcube Server and visualize with xcube Viewer
3. See error

Expected behavior
I expect int16 to be a valid value encoding and to be served and visualized without problems.

Additional context
xcube Viewer version 1.7.1
xcube Server version 1.13.1

[pont-us]

Note: from inspection of xcube.util.jsonencoder._convert_default, it looks as though this should already be handled:

            elif np.issubdtype(obj.dtype, np.integer):
                return int(obj)

np.issubdtype(np.int16, np.integer) evaluates to True, so this should be casting the integer to a Python int, making it JSON-serializable. At present, I don't know why this is not working as expected.