Fix unmarshal anyOf from a json string

[nhatthm]

Description

Fix unmarshal anyOf from a json string

[codecov]

Codecov Report

Merging #53 (e9a6331) into master (db4a8d1) will decrease coverage by 0.37%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   97.56%   97.19%   -0.37%     
==========================================
  Files          45       45              
  Lines        2383     2392       +9     
==========================================
  Hits         2325     2325              
- Misses         49       55       +6     
- Partials        9       12       +3     

Flag	Coverage Δ
unittests-Linux	97.19% <66.66%> (-0.37%)	⬇️
unittests-macOS	97.19% <66.66%> (-0.37%)	⬇️

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Impacted Files	Coverage Δ
request/stream_step.go	88.46% <60.00%> (-11.54%)	⬇️
request/client_stream.go	100.00% <100.00%> (ø)
request/server_stream.go	100.00% <100.00%> (ø)
request/unary.go	100.00% <100.00%> (ø)

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