`token_required` decorator does not work correctly on instance methods

[jkorinth]

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The current_user is passed into the decorated function first, switching places with self:

https://github.com/app-generator/api-server-flask/blob/748432027dcdb512cf76a6b45100db2c2c3ef1eb/api/routes.py#L76

A possible fix would be to retain the first param in the inner decorator function:

def token_required(f):
    @wraps(f)
    def decorator(ref, *args, **kwargs):
         ...
         # call with ref
         return f(ref, current_user, *args, **kwargs)

But this will fail on regular functions. Probably inspect could be used to make it general.