Add tests for item validation pipeline

[VMRuiz]

The current spidermon behavior for adding _validation field to items seems to be a bit controversial: #379

To make easier to track changes, I have implemented some additional tests that covers the current logic.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.12% 🎉

Comparison is base (a75cf97) 78.52% compared to head (1347209) 78.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #414      +/-   ##
==========================================
+ Coverage   78.52%   78.64%   +0.12%     
==========================================
  Files          73       73              
  Lines        3152     3152              
  Branches      528      528              
==========================================
+ Hits         2475     2479       +4     
+ Misses        600      598       -2     
+ Partials       77       75       -2     

see 2 files with indirect coverage changes

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[VMRuiz]

The error raised in these lines https://github.com/scrapinghub/spidermon/pull/414/files#diff-877bfbf0203e928b3c50d4dadb782369861cf83240c9d0721ed1f7dd791a6406R140-R145 may be a bug.

Spidermon should allow setting the default value in classes that supports using additional fields like dicts but that doesn't support __setitem__ method. I can fix it by using setattr(item, self.errors_field, defaultdict(list) instead of item[self.errors_field] = defaultdict(list). However I prefer to merge this first and fix it in a new PR.

Also, it should check that data[self.errors_field] is not None to cover the cases where the field was defined but was given a None value as default.