Fix for key not being saved with custom fields on services

[apricot13]

Previously we stopped the custom fields being initialised by both key and label but that meant that key now wasn't being set when services were saved.

This PR:

• Includes the current value of key when a custom field is saved
• Adds in some more tests around the custom fields

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.41%. Comparing base (fb841aa) to head (5eff51b).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #390   +/-   ##
========================================
  Coverage    57.41%   57.41%           
========================================
  Files          125      125           
  Lines         2898     2898           
========================================
  Hits          1664     1664           
  Misses        1234     1234           

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