fix(mapping): fix mapping when using schema class

[sbrieuc]

Fix for #66

Use of issubclass instead of isinstance to map types.

[codecov-io]

Codecov Report

Merging #70 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
+ Coverage   97.78%   97.82%   +0.04%     
==========================================
  Files           6        6              
  Lines         766      781      +15     
==========================================
+ Hits          749      764      +15     
  Misses         17       17              

Impacted Files	Coverage Δ
flask_accepts/utils.py	100.00% <100.00%> (ø)
flask_accepts/utils_test.py	100.00% <100.00%> (ø)

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[apryor6]

Looks good, thank you very much!

[apryor6]

Available in 0.16.5 via pip