Preserve card metadata format/ordering on load->save

[hlky]

This PR ensures the format/ordering of card metadata is preserved on load->save.

Example:

from huggingface_hub import ModelCard

model_card = """---
license: mit
datasets:
- foo
- bar
---

Hello
"""

card = ModelCard(model_card)
card.data.license = "test"
card.content

Currently produces:

---
datasets:
- foo
- bar
license: test
---

Hello

Now:

---
license: test
datasets:
- foo
- bar
---

Hello

Fixes #2564

[Wauplin]

Thank you @hlky for the contribution! Looks good to me now :)

[HuggingFaceDocBuilderDev]

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[hlky]

Tests should be fixed.

[Wauplin]

Failed test is unrelated :) Let's merge this!