Add test_generic_get_bytes

[oittaa]

Some serializers, like JSON, don't support byte strings and those need to be handled as a special case. It might be useful to test for that anyway.

[codecov-io]

Codecov Report

Merging #236 (6ace510) into master (3eddad1) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
+ Coverage   85.28%   85.31%   +0.02%     
==========================================
  Files          21       21              
  Lines        2624     2629       +5     
==========================================
+ Hits         2238     2243       +5     
  Misses        386      386              

Flag	Coverage Δ
unittests	85.31% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
tests/test_backend_cache.py	93.96% <100.00%> (+0.07%)	⬆️
flask_caching/__init__.py	88.29% <0.00%> (+0.06%)	⬆️

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[sh4nks]

Thanks!