Fix float64 truncation error

[ProfessorMc]

Issue: #289

Fixed it, non-breaking. Updated tests to add the new 64bit float test cases.

[codecov]

Codecov Report

Merging #293 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
+ Coverage   92.17%   92.19%   +0.02%     
==========================================
  Files           9        9              
  Lines        1892     1897       +5     
==========================================
+ Hits         1744     1749       +5     
  Misses        103      103              
  Partials       45       45

Impacted Files	Coverage Δ
tomltree_write.go	86.05% <100%> (+0.24%)	⬆️

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[pelletier]

Thank you very much for your patch! Didn't know math/big could help check for representation precision. Seems to me that this fixes the issue nicely.