fix: improve performance on saving models

[TurnrDev]

Use update_fields on save when used in a limited context. Also prevents other potentially unwanted data from being saved.

[codecov[bot]]

Codecov Report

Merging #119 (0c30883) into master (2f0db69) will not change coverage. The diff coverage is 83.33%.

:exclamation: Current head 0c30883 differs from pull request most recent head eced5a0. Consider uploading reports for the commit eced5a0 to get more accurate results

@@           Coverage Diff           @@
##           master     #119   +/-   ##
=======================================
  Coverage   64.98%   64.98%           
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  Files          28       28           
  Lines        1411     1411           
=======================================
  Hits          917      917           
  Misses        494      494           

Impacted Files	Coverage Δ
src/oscar_accounts/abstract_models.py	86.82% <83.33%> (ø)

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[TurnrDev]

I've had to fix a test to work with this change. This means that this performance improvement is a potentially breaking change so I've added a note in the docstrings about this.

[TurnrDev]

@solarissmoke All fixed! Life really hit hard but all sorted

[TurnrDev]

Nice, thank you!