[fix] ensure ref-looking scalars are quoted

[biojppm]

fixes #151

[codecov]

Codecov Report

Merging #156 (7570eff) into master (ecc32d5) will increase coverage by 0.23%.
The diff coverage is 99.33%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   95.97%   96.20%   +0.23%     
==========================================
  Files          63       63              
  Lines       11997    12232     +235     
==========================================
+ Hits        11514    11768     +254     
+ Misses        483      464      -19     

Impacted Files	Coverage Δ
src/c4/yml/parse.hpp	98.55% <ø> (ø)
test/test_double_quoted.cpp	100.00% <ø> (ø)
test/test_plain_scalar.cpp	100.00% <ø> (ø)
test/test_simple_doc.cpp	100.00% <ø> (ø)
test/test_single_quoted.cpp	100.00% <ø> (ø)
src/c4/yml/tree.hpp	98.85% <88.23%> (-0.56%)	⬇️
src/c4/yml/emit.def.hpp	96.08% <100.00%> (+2.89%)	⬆️
src/c4/yml/emit.hpp	100.00% <100.00%> (ø)
src/c4/yml/node.hpp	97.61% <100.00%> (+0.11%)	⬆️
src/c4/yml/parse.cpp	92.64% <100.00%> (+0.63%)	⬆️
... and 4 more

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