CustomField: Add a recursiveMapping function that fixes key errors wi…

[jpuerto-psc]

…th CustomFields that have nested Arrays.

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Description

This PR fixes a bug where CustomFields that have nested Arrays fail to publish. This failure is due to the "_key" attribute that does not get removed as part of the serialization.

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[jpuerto-psc]

@zzacharo - Please review at your convenience.