fix: implement get on out of order container proxy

[mbelang]

fixes: #80

[mbelang]

@sdispater please :)

[codecov-io]

Codecov Report

Merging #81 into master will decrease coverage by 0.11%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
- Coverage   91.11%   90.99%   -0.12%     
==========================================
  Files          10       10              
  Lines        2206     2211       +5     
==========================================
+ Hits         2010     2012       +2     
- Misses        196      199       +3

Impacted Files	Coverage Δ
tomlkit/container.py	84.24% <20%> (-0.49%)	⬇️

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