Challenge 3: -i vs. -v — Possible Solution
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They should reach for -v (verbose), not -i.

-i only reveals information about the RESPONSE — the status line and
response headers, then the body. It shows nothing about what the
REQUEST itself actually contained, which is exactly what's in question
here: confirming what Content-Type their own script actually sent.

-v shows the FULL exchange in both directions — every request header
curl actually transmitted (prefixed with >), and every response header
received (prefixed with <). This is the only one of the two flags that
can directly answer "what did my script really send," since it
prints the real outgoing request exactly as it left the machine —
catching cases where the script's intended header didn't actually make
it into the request (a typo in the header name, a library silently
overriding it, or the exact -d-without-explicit-Content-Type mistake
this chapter's own gotcha described).