Fix Rubocop warnings from the todo list

[andrykonchin]

Changes:

• fixed Rubocop warnings locked in the .rubocop_todo.yml file
• refactored specs to check actual request body instead of relying on RSpec method expectations
• revised and merged spec/dynamoid/criteria_new_spec.rb and spec/dynamoid/criteria_spec.rb files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.07%. Comparing base (119628e) to head (6a9649b).
⚠️ Report is 37 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1035      +/-   ##
==========================================
+ Coverage   93.05%   93.07%   +0.02%     
==========================================
  Files          84       84              
  Lines        4448     4448              
==========================================
+ Hits         4139     4140       +1     
+ Misses        309      308       -1     

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[github-actions]

Package	Line Rate	Health
dynamoid	93%	➖
Summary	93% (4140 / 4448)	➖

Minimum allowed line rate is 90%