Fix read and write of generic XML if they are BHoMObjects

[FraserGreenroyd]

Fixes #586
Fixes #587

Issues found as part of ongoing developing in SAP_Toolkit.

[FraserGreenroyd]

@BHoMBot check compliance
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[bhombot-ci]

@FraserGreenroyd to confirm, the following actions are now queued:

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[bhombot-ci]

The check code-compliance has already been run previously and recorded as a successful check. This check has not been run again at this time.

[bhombot-ci]

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[Tom-Kingstone]

Looks good, and fixes issues as stated.

[Tom-Kingstone]

@BHoMBot check ready-to-merge

[bhombot-ci]

@Tom-Kingstone to confirm, the following actions are now queued:

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