service: fix PushFileService exception

[costapp]

ERROR! [pid 25135] 21:10:56.284733 E mitogen.ctx.ssh.35.200.203.48: mitogen: While calling no-reply method PushFileService.forward
Traceback (most recent call last):
  File "master:/home/dmw/src/mitogen/mitogen/service.py", line 260, in _invoke
    ret = method(**kwargs)
  File "master:/home/dmw/src/mitogen/mitogen/service.py", line 718, in forward
    self._forward(path, context)
  File "master:/home/dmw/src/mitogen/mitogen/service.py", line 633, in _forward
    stream = self.router.stream_by_id(context.context_id)
AttributeError: 'unicode' object has no attribute 'context_id'
^C [ERROR]: User interrupted execution

mitogen/service.py

@@ -715,7 +715,7 @@ class PushFileService(Service):
         if path not in self._cache:
             LOG.error('%r: %r is not in local cache', self, path)
             return
-        self._forward(path, context)
+        self._forward(context, path)
 
 
 class FileService(Service):