Revert "workaround boost.span non-SFINAE friendly constructor"

[ashtum]

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[cppalliance-bot]

GCOVR code coverage report https://185.http-proto.prtest.cppalliance.org/gcovr/index.html
LCOV code coverage report https://185.http-proto.prtest.cppalliance.org/genhtml/index.html
Coverage Diff https://185.http-proto.prtest.cppalliance.org/gcovr/coverage_diff.txt

Build time: 2025-10-24 09:57:55 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.50%. Comparing base (1a8a81c) to head (d5acb82).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #185      +/-   ##
===========================================
+ Coverage    92.48%   92.50%   +0.02%     
===========================================
  Files           71       71              
  Lines         4854     4858       +4     
===========================================
+ Hits          4489     4494       +5     
+ Misses         365      364       -1     

Files with missing lines	Coverage Δ
include/boost/http_proto/impl/sink.hpp	95.45% <100.00%> (-4.55%)	⬇️
include/boost/http_proto/impl/source.hpp	100.00% <100.00%> (ø)
include/boost/http_proto/sink.hpp	100.00% <100.00%> (ø)
include/boost/http_proto/source.hpp	100.00% <100.00%> (ø)
src/sink.cpp	100.00% <ø> (+7.69%)	⬆️
src/source.cpp	94.11% <ø> (+5.88%)	⬆️

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