dkim: converted classes to ES6

[CalgaryMichael]

Fixes #2133

Changes proposed in this pull request:

• Convert the current classes in dkim module to ES6

Checklist:

• docs updated
• tests updated
• Changes updated

[codecov]

Codecov Report

Merging #2206 into master will decrease coverage by 0.03%.
The diff coverage is 71.22%.

@@            Coverage Diff             @@
##           master    #2206      +/-   ##
==========================================
- Coverage   59.55%   59.52%   -0.04%     
==========================================
  Files          28       28              
  Lines        6043     6043              
  Branches     1496     1496              
==========================================
- Hits         3599     3597       -2     
- Misses       2444     2446       +2

Impacted Files	Coverage Δ
dkim.js	75% <71.22%> (-0.61%)	⬇️

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[msimerson]

LGTM