fix KeyError when server reports unknown collation

[ark-lfz]

see #592

[codecov]

Codecov Report

Merging #289 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #289   +/-   ##
=======================================
  Coverage   92.56%   92.56%           
=======================================
  Files           9        9           
  Lines        1103     1103           
  Branches      156      156           
=======================================
  Hits         1021     1021           
  Misses         57       57           
  Partials       25       25

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[jettify]

thanks!

[jettify]

New version available https://pypi.org/project/aiomysql/0.0.15/