Fix failing Elixir AAI token refresh

Refreshing Elixir AAI tokens fails with error `An error occurred when refreshing user token: 401 Client Error: 401 for url: https://login.elixir-czech.org/oidc/token`. The source of the HTTP 401 error is a malformed request.

social-auth-core is sending this sort of request (a few headers are omitted)

```
POST /oidc/token HTTP/1.1
Host: login.elixir-czech.org
Accept: application/json
Content-Type: application/x-www-form-urlencoded
Content-Length: ***

{"grant_type": "refresh_token", "refresh_token": "******", "client_id": "******", "client_secret": "******"}
```

and getting this sort of response (again some headers are omitted).

```
HTTP/1.1 401 401
Content-Type: application/json

{"error":"invalid_client","error_description":"Bad client credentials"}
```

If the requests are modified so that they look like it is described in RFC6749 (https://datatracker.ietf.org/doc/html/rfc6749#section-2.3.1)

```
POST /oidc/token HTTP/1.1
Host: login.elixir-czech.org
Accept: application/json
Content-Type: application/x-www-form-urlencoded
Content-Length: ***

grant_type=refresh_token&refresh_token=******&client_id=******&client_secret=******
```

then everything works smoothly.

Author: kysrpex

social_core/backends/elixir.py

@@ -32,3 +32,14 @@ def get_user_details(self, response):
             "first_name": first_name,
             "last_name": last_name,
         }
+
+    def refresh_token(self, token, *args, **kwargs):
+        params = "&".join(
+            f"{param}={value}"
+            for param, value in self.refresh_token_params(token, *args, **kwargs).items()
+        )
+	    url = self.refresh_token_url()
+        method = self.REFRESH_TOKEN_METHOD
+        key = "params" if method == "GET" else "data"
+        request_args = {"headers": self.auth_headers(), "method": method, key: params}
+        return self.process_refresh_token_response(request, *args, **kwargs)