[fix] Wallabag - Use JSON.encode to encode auth JSON (#4449)

Previously the plugin was creating JSON by hand for authentication requests.
This would cause invalid JSON to be sent when the password contained characters
that need to be escaped (e.g. "). Using JSON.encode will result in values being
properly escaped when necessary.

plugins/wallabag.koplugin/main.lua

@@ -261,20 +261,22 @@ function Wallabag:getBearerToken()
 
     local login_url = "/oauth/v2/token"
 
-    local body = string.format( [[{
-"grant_type": "password",
-"client_id": "%s",
-"client_secret": "%s",
-"username": "%s",
-"password": "%s"
-}]], self.client_id, self.client_secret, self.username, self.password )
+    local body = {
+      grant_type = "password",
+      client_id = self.client_id,
+      client_secret = self.client_secret,
+      username = self.username,
+      password = self.password
+    }
+
+    local bodyJSON = JSON.encode(body)
 
     local headers = {
         ["Content-type"] = "application/json",
         ["Accept"] = "application/json, */*",
-        ["Content-Length"] = tostring(#body),
+        ["Content-Length"] = tostring(#bodyJSON),
         }
-    local result = self:callAPI( "POST", login_url, headers, body, "" )
+    local result = self:callAPI( "POST", login_url, headers, bodyJSON, "" )
 
     if result then
         self.access_token = result.access_token