Prevent prototype pollution of returned diff object

[mattphillips]

Fixes #85

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (55f9c3c) compared to base (ba84464).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #87   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          109       102    -7     
  Branches        36        36           
=========================================
- Hits           109       102    -7     

Impacted Files	Coverage Δ
src/added.js	100.00% <100.00%> (ø)
src/deleted.js	100.00% <100.00%> (ø)
src/diff.js	100.00% <100.00%> (ø)
src/updated.js	100.00% <100.00%> (ø)
src/utils.js	100.00% <100.00%> (ø)

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[Retr02332]

According to my tests, I can no longer contaminate the prototype:

[Retr02332]

Since the library was the one who executed the vulnerable action, it must be the one who validates the prototype. Therefore, if it is a security flaw.

Thank you very much for fixing this problem.