umqtt.simple: multiple subscribe() need check_msg()

[puuu]

Example script:

import time
from umqtt.simple import MQTTClient

def sub_cb(topic, msg):
    print((topic, msg))

def main(server="localhost"):
    c = MQTTClient("umqtt_client", server)
    c.set_callback(sub_cb)
    c.connect()
    c.subscribe(b"foo_topic")
    #time.sleep_ms(100)
    #c.check_msg()
    c.subscribe(b"bar_topic")
    while True:
        c.wait_msg()

    c.disconnect()

Running with mosquitto MQTT broker seems to work fine with:

$ mosquitto_pub -t "foo_topic" -m "1"
$ mosquitto_pub -t "bar_topic" -m "1"
$ mosquitto_pub -t "bar_topic" -m "3"

However, having a retained message,

$ mosquitto_pub -t "foo_topic" -m "1" -r

the start of the script fails with:

Traceback (most recent call last):
  File "example_sub_multi.py", line 21, in <module>
  File "example_sub_multi.py", line 14, in main
  File "/home/frederik/Downloads/micropython-lib/umqtt.simple/umqtt/simple.py", line 104, in subscribe
AssertionError:

Having a look in the code shows that subscribe() is expecting the
answer of the second subscribe (0x90), but since the message of the
first subscribe arrived, the AssertionError is thrown. So,
uncommenting sleep_ms() and check_msg() results in a working
script.

@pfalcon: This is a bug or by intention?

[pfalcon]

This is definitely a bug, feel free to propose a patch.

[puuu]

Solved with #89