API - Add support for request content in JSON

[Josue-T]

Problem

Using yunhost admin API with as REST API has not issue.

By example when we use it with the requests lib passing the parameter is straightforward. And I didn't find a working solution to pass correctly an UTF-8 easily with requests.

Solution

Add a working support for REST request with JSON content type.

Note that for now the authentication is not supported by this way. But the issue currently is on the other request which could be difficult to write to make it working correctly.

How to test

By example we requests we can test this way:

import requests
s = requests.Session()
s.headers['X-Requested-With'] = 'Client'
s.post("https://domain.tld/yunohost/api/login", data={"credentials": "%s:%s" % ("user", "pwd")})
s.post("https://domain.tld/yunohost/api/users", json={"username": "yolotest", "fullname": "Fullname Withspécialchàr", "password": "pwd", "domain": "domain.tld"})