Fix IProgress<T> when T is a MessagePack [Union] type

[AArnott]

Fixes #537

[codecov-commenter]

Codecov Report

Merging #540 into master will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #540      +/-   ##
==========================================
- Coverage   89.76%   89.70%   -0.07%     
==========================================
  Files          53       53              
  Lines        4408     4410       +2     
==========================================
- Hits         3957     3956       -1     
- Misses        451      454       +3     

Impacted Files	Coverage Δ
...nRpc/Reflection/MessageFormatterProgressTracker.cs	94.80% <100.00%> (+0.13%)	⬆️
src/StreamJsonRpc/JsonRpc.cs	92.48% <0.00%> (-0.32%)	⬇️

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