ToElement methods do not export data from custom xml namespaces

## Environment
* OS Version: any
* Source or binary build? any

## Description
* Expected behavior: if you have an SDFormat file with data in custom XML namespaces (like [custom_elems_attrs.sdf](https://github.com/gazebosim/sdformat/blob/sdf13/test/integration/custom_elems_attrs.sdf)), if you load the file into an `sdf::Root` and then write it out again with `Root::ToElement()`, the resulting Element-tree should also have the custom data fields.
* Actual behavior: it does not include the custom data fields.

## Steps to reproduce

1. TODO: write code to reproduce this
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## Output

Use test/integration/custom_elems_attrs.sdf as the input and inspect the Root::ToElement() call path. Add a focused reproduction for loading that file and converting it back to an Element-tree. Done means custom XML namespace data is present in the resulting tree.

Written by the indexing model from the issue text.