Ensure serializers are instances

[argaen]

Closes #377

[codecov]

Codecov Report

Merging #379 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #379   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines         844    853    +9     
  Branches       89     89           
=====================================
+ Hits          844    853    +9

Impacted Files	Coverage Δ
aiocache/backends/memcached.py	100% <ø> (ø)	⬆️
aiocache/backends/memory.py	100% <ø> (ø)	⬆️
aiocache/backends/redis.py	100% <ø> (ø)	⬆️
aiocache/base.py	100% <ø> (ø)	⬆️
aiocache/serializers.py	100% <100%> (ø)	⬆️

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