Fix to_chars_hex exp for Float128 types

[mborland]

Closes: #149

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8f5dde7) 88.53% compared to head (0716ed2) 88.53%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #151   +/-   ##
========================================
  Coverage    88.53%   88.53%           
========================================
  Files           59       59           
  Lines         8033     8033           
========================================
  Hits          7112     7112           
  Misses         921      921           

Files	Coverage Δ
src/to_chars_float_impl.hpp	96.25% <ø> (ø)
test/test_float128.cpp	100.00% <ø> (ø)

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