Fix retrieving Twitter conversations

Summary:
Fix a regression from 0ecc05c1 where choqok can no longer retrieve Twitter conversations due to a mismatch between the authorization header and the actual post url

This was already fixed for requestTimeLine in f8285c35 but not for fetchPost

BUG: 417193

Test Plan: Clicked on "Display conversation" in a tweet from the timeline, no more authorization errors and the conversation is displayed correctly

Reviewers: scarpino

Reviewed By: scarpino

Differential Revision: https://phabricator.kde.org/D27236