Fix attributes casting for CVAT format

[sizov-kirill]

Summary

Fixed #395.

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[zhiltsov-max]

Add a test for number attributes, now there is no such tests.

[sizov-kirill]

Add a test for number attributes, now there is no such tests.

I changed my solution, deleted number type because to fully support it we need to implement data model for attributes.

[zhiltsov-max]

So what about the test? I don't see how your comment is related to this.

[zhiltsov-max]

Please update the changelog