Rename last_will method -> will_set

[brentru]

Keeping in-line with Paho-MQTT, we may want to change last_will -> will_set. It's more descriptive and defines the method better.

Paho Implementation: will_set(topic, payload=None, qos=0, retain=False)

Our Implementation:
https://github.com/adafruit/Adafruit_CircuitPython_MiniMQTT/blob/master/adafruit_minimqtt.py#L192

[brentru]

Additionally, LWT should not be called within init since LWT should be set independently prior to connecting.

[brentru]

payload also needs to be converted into bytes, checking does not occur within last_will, causing a failure when attempting to fix.

calling: client.will_set("devices/{}/status".format(client_id), payload=1, retain=True)
error:

SET LWT
Attempting to connect to SNIP!
Traceback (most recent call last):
  File "code.py", line 94, in <module>
  File "/lib/adafruit_minimqtt.py", line 263, in connect
TypeError: object of type 'int' has no len()