ValidataclassMixin.to_dict: Optionally filter None values

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Currently the to_dict() method of the ValidataclassMixin has no option to automatically filter None values, only for filtering UnsetValue (which it does by default but can be disabled by setting keep_unset_values=True).

There should be an option to filter None values (default: don't filter). The parameter could be called filter_none.

For consistency, the keep_unset_values parameter MAYBE should be renamed to filter_unset_value (default: True) as well, but this would be a breaking change in the API, so the existing parameter should be kept as an alias (with a deprecation warning) and later removed in the 1.0.0 release.

(This will potentially be included in #62.)