ref: make user.other mutable

[bruno-garcia]

Resolves #34

[codecov-io]

Codecov Report

Merging #35 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   94.75%   94.74%   -0.01%     
==========================================
  Files          26       26              
  Lines         648      647       -1     
==========================================
- Hits          614      613       -1     
  Misses         34       34

Impacted Files	Coverage Δ
src/Sentry.Protocol/User.cs	100% <100%> (ø)	⬆️

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