Allow invoking of to_dict (and others) on default sget arguments

[rszalski]

The common pattern I see is:

if reply.sth:
    return reply.sth.to_dict()
return {}

It would be nice if sget() supported returning of default argument on which you could still invoke to_dict():

return reply.sget('sth', {}).to_dict()

But this might be tricky to implement, since default argument gets returned as is. In this case, Pythons dict do not support to_dict().
Alternative would be to specify helper methods directly in the sget() string:

return reply.sget('sth.to_dict()', {})