fix: pass the body as string regardless of it is json

[teto]

fix: pass the body as string regardless of it is json else we hit errors in plenary.

In Plenary we can set the "body" as a string (either filename or the actual content of the payload)
or as a table in which case it is passed as curl --data . According to the doc, the latter does:

        -d, --data <data>
                  (HTTP MQTT) Sends the specified data in a POST request to the HTTP server, in the same way that a browser does when a user has filled in an HTML form and presses the submit button. This will cause curl to pass the data to the server using the content-type application/x-www-form-urlencoded. Compare
                  to -F, --form.

Currently rest.nvim sends a table when the file is a json one which makes no sense as per curl doc so always send it as a string (aka as "content")

The relevant plenary code;
https://github.com/nvim-lua/plenary.nvim/blob/1da13add868968802157a0234136d5b1fbc34dfe/lua/plenary/curl.lua#L183-L197

[teto]

btw looking at https://www.reddit.com/r/neovim/comments/ug9ypl/restnvim_is_looking_for_maintainerscontributors/ , I dont really want to get involved with the development but I wouldn't mind merging rights to be able to merge the occasionnal fixes.

[teto]

It's possible to see the issue using the following json file

{
    "_categoricalParams": [
        {
            "allLevels": [
                "one",
                "three",
                "two"
            ],
            "currentLevel": "one",
            "description": "",
            "name": "cat_param1",
            "reference": ""
        },
        {
            "allLevels": [
                "one",
                "three",
                "two"
            ],
            "currentLevel": "one",
            "description": "",
            "name": "cat_param2",
            "reference": ""
        },
        {
            "allLevels": [
                "one",
                "three",
                "two"
            ],
            "currentLevel": "one",
            "description": "",
            "name": "cat_param3",
            "reference": ""
        }
    ]
	}

Without this PR, I get:

[rest.nvim] Failed to perform the request.                                                                                                                                                                                                                                                                                      
Make sure that you have entered the proper URL and the server is running.                                                                                                                                                                                                                                                       
Traceback: /home/teto/neovim/plenary.nvim/lua/plenary/curl.lua:57: attempt to concatenate a table value  

[teto]

apparently fixed by #125 (haven't tested)