Fix jws.Sign() to allow detached, unencoded payload

[lestrrat]

fixes #541

[codecov-commenter]

Codecov Report

Merging #543 (177685a) into main (5ded25f) will increase coverage by 0.03%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main     #543      +/-   ##
==========================================
+ Coverage   72.82%   72.85%   +0.03%     
==========================================
  Files          84       84              
  Lines        9490     9508      +18     
==========================================
+ Hits         6911     6927      +16     
- Misses       1765     1767       +2     
  Partials      814      814              

Impacted Files	Coverage Δ
jws/option.go	76.47% <33.33%> (-10.20%)	⬇️
jws/jws.go	77.97% <100.00%> (+0.45%)	⬆️
jws/message.go	82.37% <100.00%> (+0.64%)	⬆️

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