panel.serve only accepts functions, not methods

[nritsche]

panel version info

0.9.6

Description of expected behavior and the observed behavior

server = MyServer()
panel.serve(server.gui_instance)

Should serve a template returned by MyServer.gui_nstance(), but it only produces a > int he browser and no error message.

Replaced by the following, it produces the expected behavior:

server = MyServer()
def instance():
    return server.gui_instance()
panel.serve(instance)