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 @BHoMBot check required

[bhombot-ci[bot]]

@FraserGreenroyd to confirm, the following actions are now queued: - check `code-compliance` - check `documentation-compliance` - check `project-compliance` - check `branch-compliance` - check `dataset-compliance` - check `copyright-compliance` - check `code-compliance` - check `documentation-compliance` - check `project-compliance` - check `core` - check `null-handling` - check `serialisation` - check `versioning` - check `installer`

[bhombot-ci[bot]]

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[bot]]

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

[Tom-Kingstone]

@BHoMBot check ready-to-merge

[bhombot-ci[bot]]

@Tom-Kingstone to confirm, the following actions are now queued: - check `ready-to-merge`