Fix out of order tables dict-like behavior

[sdispater]

Fixes #78

[codecov-io]

Codecov Report

Merging #79 into master will decrease coverage by 0.06%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
- Coverage   91.18%   91.11%   -0.07%     
==========================================
  Files          10       10              
  Lines        2200     2206       +6     
==========================================
+ Hits         2006     2010       +4     
- Misses        194      196       +2

Impacted Files	Coverage Δ
tomlkit/container.py	84.73% <71.42%> (-0.25%)	⬇️

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