I recently had need of a service that would return your exact query to you for web client (or proxy) development purposes, in a case where using Fiddler, Wireshark, tcpdump et.al. was proving difficult for various reasons. The solution became the C-Net Echo Service, available at https://echo.c-net.org/ (and over plain HTTP if you need it). Feel free to use this for whatever.
$ curl -X PUT -d "Hello World!" -H "User-Agent: Fake Agent" -H "CUSTOM-HEADER: foo" -H "X-Forwarded-For: 188.8.131.52" "https://echo.c-net.org/"
PUT / HTTP/1.1
User-Agent: Fake Agent