fix Value::to_json order of items in array

[sele9]

This Pull Request fixes/closes #690.

It changes the following:

• preserve order of array items on Value::to_json

[codecov]

Codecov Report

Merging #699 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #699      +/-   ##
==========================================
+ Coverage   71.69%   71.73%   +0.03%     
==========================================
  Files         199      199              
  Lines       13865    13866       +1     
==========================================
+ Hits         9941     9947       +6     
+ Misses       3924     3919       -5     

Impacted Files	Coverage Δ
boa/src/value/mod.rs	69.75% <100.00%> (+0.07%)	⬆️
boa/src/property/mod.rs	49.65% <0.00%> (-0.70%)	⬇️
boa/src/object/iter.rs	20.20% <0.00%> (+6.06%)	⬆️

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