Fix UnboundLocalError in push() and clonedb() when no remote auth configured

`headers` was only initialized inside the `if self._remote_auth_dict or
remote_auth:` block, but `headers.update(self._default_headers)` ran
unconditionally. When neither remote_auth parameter nor client-level
remote auth is set (the common case for local/unauthenticated connections),
this raises `UnboundLocalError: cannot access local variable 'headers'`.

Fix: initialize `headers = {}` before the conditional block so the
subsequent `.update()` always has a valid dict to work with.

Affected methods:
- `Client.push()`
- `Client.clonedb()`

Note: `Client.pull()` is not affected — it passes `self._default_headers`
directly without the conditional pattern.