Issue in notify websocket

[mellertson]

After following the following example, found in the official docs, I noticed the callback method def message(self, reply, *args, **kwargs) in class BitSharesWebsocket(Events) would consistently raise and thus never print data to standard out.

Example

from pprint import pprint
from bitshares.notify import Notify
from bitshares.market import Market

notify = Notify(
    markets=["TEST:GOLD"],
    accounts=["xeroc"],
    on_market=print,
    on_account=print,
    on_block=print,
    on_tx=print
)
notify.listen()

I looked for a test case of the the Notify class, but didn't find one. After writing a makeshift integration test, which sadly is not sufficient to commit to the repo, I found the issue and corrected it in this PR. I have seen this kind of issue previously. It looked to me like the underlying websocket.WebSocketApp class is intended to be used as a base class. But, I can't be 100% about that assumption.

Regardless, the code I am submitting in this PR does resolve the issue and was written in a way such that it should not impact other code.

P.S.

Instead of rebasing, as you suggested here, I just created this new PR.

[CLAassistant]

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