Improve coverage of frame_header.hpp

[djarek]

No description provided.

[codecov]

Codecov Report

Merging #13 into master will increase coverage by 10.25%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #13       +/-   ##
=========================================
+ Coverage   89.74%   100%   +10.25%     
=========================================
  Files           5      5               
  Lines          78     78               
=========================================
+ Hits           70     78        +8     
+ Misses          8      0        -8

Impacted Files	Coverage Δ
include/canary/frame_header.hpp	100% <0%> (+28.57%)	⬆️

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