fix: method "keys" return an immutable copy of the cache keys

[rafamizes]

Closes #165

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.52%. Comparing base (27b77a1) to head (6c23f0d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #166   +/-   ##
=======================================
  Coverage   99.52%   99.52%           
=======================================
  Files          11       11           
  Lines         210      210           
=======================================
  Hits          209      209           
  Misses          1        1           

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