Add deep copy of Difficulty field in types.CopyHeader

[chetna-mittal]

Description

Add a deep copy of the Difficulty field in types.CopyHeader function.

Resolves: #2309

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.94%. Comparing base (f0adb14) to head (5fd0de0).
Report is 57 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2353      +/-   ##
==========================================
- Coverage   55.06%   54.94%   -0.12%     
==========================================
  Files         684      685       +1     
  Lines      114596   114965     +369     
==========================================
+ Hits        63097    63164      +67     
- Misses      47617    47895     +278     
- Partials     3882     3906      +24     

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[palango]

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