test: JsonCacheWrap, JsonCacheMem and JsonCacheFake.

[rafamizes]

Closes #22

[codecov]

Codecov Report

Merging #23 (d89a8e1) into master (5730204) will increase coverage by 79.54%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           master       #23       +/-   ##
============================================
+ Coverage   20.45%   100.00%   +79.54%     
============================================
  Files           3         3               
  Lines          44        44               
============================================
+ Hits            9        44       +35     
+ Misses         35         0       -35     

Impacted Files	Coverage Δ
lib/src/json_cache_fake.dart	100.00% <100.00%> (+80.00%)	⬆️
lib/src/json_cache_mem.dart	100.00% <100.00%> (+72.00%)	⬆️
lib/src/json_cache_wrap.dart	100.00% <0.00%> (+100.00%)	⬆️

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