Unmarshal only actually read bytes

[braams]

Description

The interceptor tried to unmarshal the whole buffer and didn't respect the actual packet length.

Reference issue

Fixes #255

[thatsnotright]

Let me add some tests around this, unless you wish to, and then we can get this merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.48%. Comparing base (f42ed2b) to head (561796c).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
- Coverage   72.56%   72.48%   -0.08%     
==========================================
  Files          79       79              
  Lines        3648     3653       +5     
==========================================
+ Hits         2647     2648       +1     
- Misses        867      869       +2     
- Partials      134      136       +2     

Flag	Coverage Δ
go	72.48% <100.00%> (-0.08%)	⬇️
wasm	?

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