write_string_fragment should take &str instead of &[u8]

[dtolnay]

https://docs.serde.rs/serde_json/ser/trait.Formatter.html#method.write_string_fragment

rq is using unsafe to get a &str: https://github.com/dflemstr/rq/blob/v0.9.2/src/value/json.rs#L224

let s = unsafe { str::from_utf8_unchecked(fragment) };

It should be our responsibility to guarantee that the fragment is UTF-8.

cc @dflemstr

[dtolnay]

Frustratingly, it is such an easy fix: #269.

[dflemstr]

I approve of this 👍

[dtolnay]

Fixed in #297.