Optimise #save

[andrykonchin]

Improve saving:

• send only changed attributes when save persisted model
• don't send any request when there are no changed attributes

[codecov]

Codecov Report

Merging #623 (d2d67fc) into master (103a4c7) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head d2d67fc differs from pull request most recent head e93054b. Consider uploading reports for the commit e93054b to get more accurate results

@@            Coverage Diff             @@
##           master     #623      +/-   ##
==========================================
- Coverage   90.22%   90.18%   -0.05%     
==========================================
  Files          61       60       -1     
  Lines        3100     3076      -24     
==========================================
- Hits         2797     2774      -23     
+ Misses        303      302       -1     

Impacted Files	Coverage Δ
lib/dynamoid/components.rb	100.00% <ø> (ø)
...dynamoid/adapter_plugin/aws_sdk_v3/item_updater.rb	100.00% <100.00%> (ø)
lib/dynamoid/dirty.rb	96.42% <100.00%> (+0.04%)	⬆️
lib/dynamoid/fields.rb	100.00% <100.00%> (ø)
lib/dynamoid/persistence.rb	98.06% <100.00%> (-0.02%)	⬇️
lib/dynamoid/persistence/import.rb	100.00% <100.00%> (ø)
lib/dynamoid/persistence/save.rb	96.36% <100.00%> (-3.64%)	⬇️
lib/dynamoid/document.rb	95.12% <0.00%> (-0.44%)	⬇️
lib/dynamoid/adapter_plugin/aws_sdk_v3.rb	94.30% <0.00%> (-0.18%)	⬇️
... and 8 more

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[github-actions]

Package	Line Rate	Health
dynamoid	90%	➖
Summary	90% (2812 / 3117)	➖

Minimum allowed line rate is 90%