fix: Improper namespace application in elementToBytes

[jeremy-clerk]

In the elemToBytes function when applying namespaces to the doc.Root() it is assumed that the namespace will have a prefix. If the namespace is the default namespace xmlns the function improperly applies the namespace as:

xmlns:=<URI> - which is invalid XML.

This can cause errors when parsing Assertions that do not have explicit namespaces applied and inherit the default namespace from their parent element.

This PR updates the elemToBytes function to account for this.

Related issues:
#578
#527

[ay4toh5i]

@crewjam Hi, I wanted to mention that I’ve also been struggling with this issue, especially in cases where the default namespace is not applied correctly, leading to parsing errors. This fix seems very valuable, and I’d appreciate it if you could take a moment to review it.
Looking forward to your response. Thank you!