Fix endoding issues

[jzaprzalka]

Fixes encoding issues referenced in issue OSS-781 while running rasa data split nlu with entities containing non-english characters

Proposed changes:

• Add ensure_ascii=False for json.dumps in TrainingDataWriter in order to properly encode non-ascii chatacters, e.g. Polish letters as referenced in issue OSS-781

Status (please check what you already did):

• [ ] added some tests for the functionality
• [ ] updated the documentation
• [ ] updated the changelog (please check changelog for instructions)
• [ ] reformat files using black (please check Readme for instructions)

[CLAassistant]

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