fix: set JSON Content-Type when a single header is present

[parinporecha]

Description

When running http --print=B POST pie.dev/post 'header1: xyz' x=1 with a single custom header, HTTPie doesn't set Content-Type: application/json on the request. With two headers it works fine.

Root Cause

In httpie/client.py lines 263-275, the inner condition if args.json or (auto_json and args.data): was redundant and wrong. It prevented Content-Type from being set when args.json=False and auto_json=True with data.

Fix

Removed the redundant inner if guard. Now Content-Type is set unconditionally inside the outer if args.json or auto_json: block.

Testing

• Added test test_json_content_type_with_single_header to verify the fix
• All existing tests pass

Fixes #1640.