Don't crash on front matter with non-string mapping keys

[eeshsaxena]

frontmatter.loads crashes on a document whose front matter has a non-string key:

>>> import frontmatter
>>> frontmatter.loads("---\n1: one\n---\nbody")
TypeError: keywords must be strings

Integers, booleans and dates are all valid YAML (and TOML) mapping keys, but loads builds the post with Post(content, handler, **metadata), and splatting a dict with a non-string key into keyword arguments raises TypeError: keywords must be strings, even though the front matter itself parsed fine.

The fix assigns the parsed metadata dict to the Post directly instead of splatting it, so non-string keys are preserved and round-trip through dumps/loads. String-key documents are unaffected. Added a test.

[eeshsaxena]

Hi! Gentle nudge on this one whenever you have some bandwidth. It's a small, self-contained fix (Don't crash on front matter with non-string mapping keys), and it's currently mergeable with no conflicts. No urgency at all, and I'm happy to make any changes you'd like. Thanks for maintaining python-frontmatter!

[eeshsaxena]

Closing this to clear out my older open PRs. Nothing wrong with the change on my side, I am just tidying up a backlog. If it is still useful to you, say so and I will reopen it and rebase it on current main.