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Enhancement request

I propose a way to set a custom user-agent as a value to default_options in ~/.config/httpie/config.json. Or, perhaps adding a new config section named default_headers would be more logical.

Problem it solves

Including a custom user-agent on the command line adds about 130 characters to the input, which is a bit cumbersome if every httpie command requires one. It's a lot of extra text that doesn't change often that makes it hard to navigate the prompt fills up my history with super-long commands. Yuck.

Additional information

I actually tried to do this:

    "default_options": [
        "User-Agent:'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.3.1 Safari/605.1.15'"
    ]

…but it doesn't work because it seems the values in default_options are applied before the URL on the command line, and the user-agent (because it's a header) must be specified after the URL. Because of this, adding a default_headers config section might be easier to implement.