Handling of RQ_ASYNC is confusing when using the application factory pattern #81

When using the application factory pattern, the RQ_ASYNC configuration is only respected if it has not already been set for the task queue being initialized.

Concrete example to show the behaviour:

rq = RQ()  # rq._is_async is set to None
app = Flask(__name__)
app.config['RQ_ASYNC'] = False
rq.init_app(app)  # rq._is_async is set to False
app.config['RQ_ASYNC'] = True
rq.init_app(app)  # rq._is_async is unchanged

I'm not sure if this is a bug or not - from a look at the code, my guess is that the intention is that RQ_ASYNC does not override constructing the queue with the is_async argument set manually, which seems like a sensible stance. However, it does seem a bit strange - altering any of the other RQ_X options will take effect on a re-init.

What is the expected behavior here?