Allow callable() as field deferring mechanism

[oprypkhantc]

Closes #738

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.85%. Comparing base (53f9d49) to head (30a4f95).
Report is 111 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Mappers/CannotMapTypeException.php	75.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #739      +/-   ##
============================================
- Coverage     95.72%   94.85%   -0.87%     
- Complexity     1773     1838      +65     
============================================
  Files           154      175      +21     
  Lines          4586     4999     +413     
============================================
+ Hits           4390     4742     +352     
- Misses          196      257      +61     

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[oojacoboo]

Let's keep this test as well, instead of replacing it.