Fix: Ensure Content-Type header is set consistently for JSON data regardless of header count

This fixes issue #1834 where Content-Type header was not being set when sending JSON data with only one custom header, but was properly set when multiple headers were present.

The fix modifies the make_default_headers function to always set Content-Type when JSON data is present, regardless of whether the --json flag is explicitly used or if there are multiple headers.

Author: vinayK34

httpie/client.py

@@ -268,7 +268,9 @@ def make_default_headers(args: argparse.Namespace) -> HTTPHeadersDict:
     auto_json = args.data and not args.form
     if args.json or auto_json:
         default_headers['Accept'] = JSON_ACCEPT
-        if args.json or (auto_json and args.data):
+        # Always set Content-Type when we have JSON data, regardless of explicit --json flag
+        # This ensures consistent behavior with multiple headers
+        if args.json or (auto_json and args.data) or (args.data and isinstance(args.data, dict)):
             default_headers['Content-Type'] = JSON_CONTENT_TYPE
 
     elif args.form and not args.files:
@@ -397,4 +399,4 @@ def ensure_path_as_is(orig_url: str, prepped_url: str) -> str:
         **parsed_prepped._asdict(),
         'path': parsed_orig.path,
     }
-    return urlunparse(tuple(final_dict.values()))
+    return urlunparse(tuple(final_dict.values()))