decorators use JsonSerializer by default now

[argaen]

Also renamed DefaultSerializer to StringSerializer

Closes #252

[codecov-io]

Codecov Report

Merging #258 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #258      +/-   ##
==========================================
+ Coverage   99.18%   99.18%   +<.01%     
==========================================
  Files          13       13              
  Lines         855      856       +1     
  Branches       88       88              
==========================================
+ Hits          848      849       +1     
  Misses          7        7

Impacted Files	Coverage Δ
aiocache/settings.py	100% <ø> (ø)	⬆️
aiocache/factory.py	100% <ø> (ø)	⬆️
aiocache/backends/redis.py	100% <ø> (ø)	⬆️
aiocache/backends/memory.py	100% <ø> (ø)	⬆️
aiocache/backends/memcached.py	100% <ø> (ø)	⬆️
aiocache/decorators.py	100% <100%> (ø)	⬆️
aiocache/base.py	100% <100%> (ø)	⬆️
aiocache/serializers.py	90% <100%> (ø)	⬆️

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