Fix JSON stringification for fractional numbers

[georgeroman]

Fixes #449

[codecov]

Codecov Report

Merging #870 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #870      +/-   ##
==========================================
+ Coverage   59.03%   59.09%   +0.05%     
==========================================
  Files         164      164              
  Lines       10217    10216       -1     
==========================================
+ Hits         6032     6037       +5     
+ Misses       4185     4179       -6     

Impacted Files	Coverage Δ
boa/src/value/mod.rs	73.78% <100.00%> (+0.73%)	⬆️
...x/parser/statement/iteration/do_while_statement.rs	73.91% <0.00%> (+13.04%)	⬆️

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[Razican]

Looks good to me! Thanks :D

Build issues seem unrelated, and only happen with MacOS, not sure why.