--json with no request items omits body entirely instead of providing empty object

[lilyball]

If I do a request like xh -p HBhb POST http://example.com --json the outgoing request looks like

POST / HTTP/1.1
Accept: application/json, */*;q=0.5
Accept-Encoding: gzip, deflate, br
Connection: keep-alive
Content-Type: application/json
Host: example.com
User-Agent: xh/0.14.1

Notice the complete lack of any body despite Content-Type: application/json. This is an invalid JSON body.

In the absence of request items, passing --json should pass the body as {}.

❯ xh --version
xh 0.14.1
+native-tls

[blyxxyz]

This is more or less intentional, it matches HTTPie:

$ http --offline --json post :
POST / HTTP/1.1
Accept: application/json, */*;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Content-Length: 0
Content-Type: application/json
Host: localhost
User-Agent: HTTPie/2.6.0

It does seem wrong, so maybe we should change it. I've opened httpie/cli#1263.

(The current best way to send an actual empty JSON object is echo '{}' | xh [...].)

[lilyball]

Thank you for opening the upstream issue

[ducaale]

Given that the upstream issue has been closed with no action, I think we should go with @blyxxyz's solution i.e. don't set Content-Type unless there is a JSON body.

httpie/cli#137 already decided opposite by making explicit --json add a Content-Type header, so then maybe that should be reverted instead? Right now it's inconsistent.