Fix decimal and nil header encoding

[ptoffy]

This aims to fix the issues discovered in #188

1. nil wasn't being added because assigning nil to a field would remove it from the dict. Now assigning nil to a field actually adds an explicit null value to the json, while there's a remove(field:) method to remove fields from the header
2. decimal wasn't being decoded correctly because it was trying to unwrap Doubles as Ints

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.86%. Comparing base (5d90471) to head (e6165cc).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/JWTKit/JWTHeader.swift	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #189      +/-   ##
==========================================
+ Coverage   78.76%   78.86%   +0.09%     
==========================================
  Files          60       60              
  Lines        1413     1424      +11     
==========================================
+ Hits         1113     1123      +10     
- Misses        300      301       +1     

Files with missing lines	Coverage Δ
Sources/JWTKit/JWTHeaderField.swift	71.79% <100.00%> (+1.93%)	⬆️
Sources/JWTKit/JWTHeader.swift	89.65% <90.90%> (-1.65%)	⬇️

[fpseverino]

LGTM