Fix ambiguity in basic_stream constructor overloads

[ashtum]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.06%. Comparing base (3461ad1) to head (f26c7be).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2864      +/-   ##
===========================================
+ Coverage    93.03%   93.06%   +0.02%     
===========================================
  Files          178      178              
  Lines        13688    13688              
===========================================
+ Hits         12735    12739       +4     
+ Misses         953      949       -4     

Files	Coverage Δ
include/boost/beast/_experimental/test/stream.hpp	75.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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