Fix error which copying a subclass of Subject with keyword attributes

[c-winder]

Fixes #1181.

Updated Subject._subject_copy_helper to unpack the dictionary of attributes rather than passing it directly. This means that a subclass of Subject will correctly read any keyword arguments in the dict rather than passing the entire unpacked dict to the first argument.

test_copy_subclass has been extended to test that this works properly. The dummy subclass now has two attributes.

Checklist

• [x] I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• [x] Non-breaking change (would not break existing functionality)
• [ ] Breaking change (would cause existing functionality to change)
• [x] Tests added or modified to cover the changes
• [x] Integration tests passed locally by running pytest
• [ ] In-line docstrings updated
• [ ] Documentation updated, tested running make html inside the docs/ folder
• [x] This pull request is ready to be reviewed

[fepegar]

Thanks for your contribution, @c-winder!

@allcontributors please add @c-winder for bug

[allcontributors[bot]]

@fepegar

I've put up a pull request to add @c-winder! :tada: