Get device_id from body instead of auth data

Device_id was retrieved from the auth data instead of login's body and
this was causing that a new device was created on every login.

This is (I guess) provoking that some sytests are failing (for example,
"POST /login returns the same device_id as that in the request").

src/client_server.rs

@@ -293,6 +293,7 @@ pub fn login_route(
 
     // Generate new device id if the user didn't specify one
     let device_id = body
+        .body
         .device_id
         .unwrap_or_else(|| utils::random_string(DEVICE_ID_LENGTH));