Correctly wrap dkim header.

[wjywbs]

This change has been validated by sending large email campaigns.

[codecov-commenter]

Codecov Report

Merging #30 into master will decrease coverage by 1.40%.
The diff coverage is 46.51%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
- Coverage   66.30%   64.90%   -1.41%     
==========================================
  Files           7        7              
  Lines         837      869      +32     
==========================================
+ Hits          555      564       +9     
- Misses        213      233      +20     
- Partials       69       72       +3     

Impacted Files	Coverage Δ
dkim/header.go	75.63% <43.90%> (-17.48%)	⬇️
dkim/sign.go	68.58% <100.00%> (ø)

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[emersion]

Thanks for taking the time to write a patch, but this has been fixed in #29

[wjywbs]

Thanks. By the way, this fixes the h= wrapping as well.