fix(requesttoken): Make sure to use the correct requesttoken in WebdavClient

[SystemKeeper]

Fixes nextcloud/spreed#12529

Introduced in #2178 / #2179

Before the PR the getRequestToken was called when createClient was called, so it always contained the current value. After the PR getRequestToken is only called once when the headers object is created, so it will never change, which leads to being unable to open some pictures (see talk issue above).

[Antreesy]

Looks legit, let's wait for a second opinion

[SystemKeeper]

/backport to stable28

[SystemKeeper]

/backport to stable29

[backportbot]

The backport to stable28 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable28
git pull origin stable28

# Create the new backport branch
git checkout -b backport/2339/stable28

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick e870f7e2

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/2339/stable28

Error: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.