[shadowhand]

Summary

rw api -f ... now defaults JSON bodies to Content-Type: application/vnd.api+json. It used to send application/json, which RoundingWell API rejected.

A Content-Type passed with -H still wins, matched case-insensitively. Pass -H "Content-Type: application/json" to get the old behavior.

Accept stays at reqwest's */*, which already accepts JSON:API responses. A strict application/vnd.api+json default could break endpoints that return other media types. Callers can still pass -H "Accept: ...".

Changes

• In src/commands/api.rs, the -f body defaults to application/vnd.api+json unless -H sets Content-Type.
• New mockito tests call run() and check the Content-Type and body the server receives. They cover -f with no -H, -H "Content-Type: application/json", a mixed-case header name, and an unrelated -H Accept.
• The --field help text in src/cli.rs names the default media type.
• The README notes the default and shows the --header "Content-Type: application/json" override.

Closes CL-10

Verification

• cargo fmt --check and cargo clippy --all-targets --all-features -- -D warnings are clean.
• cargo test --locked passes, and cargo audit reports no vulnerabilities.
• Against a local echo server (--stage local), -X PUT -f data.type=jobs sent application/vnd.api+json, and adding -H "content-type: application/json" sent application/json.