Fix for #70 Check that futures were already done before call .set_exception

[rhdxmr]

This PR fixes bug reported by issue #70

@cybergrind Can you review my PR?

ps I heard the news of that Belarusian protest against the president. I am wondering that you are OK nowadays.

ps2 I had been so busy for a month so I could not spend time on aiozk project nowadays. But I expect I can do as before from now on.

[codecov-commenter]

Codecov Report

Merging #71 into master will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #71   +/-   ##
=======================================
  Coverage   88.03%   88.03%           
=======================================
  Files          51       51           
  Lines        2097     2097           
=======================================
  Hits         1846     1846           
  Misses        251      251           

Impacted Files	Coverage Δ
aiozk/connection.py	92.55% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 4d7ab03...1595068. Read the comment docs.

[cybergrind]

@rhdxmr looks good.

I heard the news of that Belarusian protest against the president.

Yeah, it looks like people unites together against the current government because the results of the election were totally fake. It is quite hard to do anything because you're either participating protests or checking news in a loop :D

I had been so busy for a month so I could not spend time on aiozk project nowadays. But I expect I can do as before from now on.

Glad to hear that. I've added coverage and reviewed most of the things that have no coverage: we're more or less good, not so many things aren't covered. But probably some recipes could be done better after we finish coverage.