fix(webdataset): don't .lower() field_name
fixes: https://github.com/huggingface/datasets/issues/7732 The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7726). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update. CI failures are unrelated, merging :)
This fixes cases where keys have upper case identifiers