Fix ElementSerializer producing invalid xmlns:= and duplicate namespace declarations

[rcbevans]

Fixes #254

Key changes

SerializeHelper::write_xmlns (xsd-parser-types/src/quick_xml/serialize.rs):

• Normalize Some(NamespacePrefix("")) to None at the top of write_xmlns, so the default namespace is correctly matched regardless of whether it was registered by generated code (None) or from Element::namespaces (empty byte slice). This also prevents add_namespace_entry from producing the invalid xmlns: attribute name.

ElementSerializer::next_item (xsd-parser-types/src/xml/element.rs):

• Normalize empty prefix to None before calling write_xmlns in the namespaces loop
• Filter xmlns attributes from element.attributes when a corresponding entry exists in element.namespaces, preventing double emission. Attributes added only via .attribute() without a matching .namespace() entry are preserved.

Tests added

Test	Scenario
roundtrip_with_default_namespace	Default namespace + prefixed namespace round-trip
roundtrip_without_namespaces	Plain elements, no xmlns
roundtrip_with_prefixed_namespace_only	Only prefixed namespace, no default
roundtrip_default_namespace_undeclaration	xmlns="" undeclares default namespace (XML Namespaces 1.0 §6.2)
roundtrip_prefix_rebinding	Child rebinds prefix to different URI (XML Namespaces 1.0 §6.1)
roundtrip_inherited_namespace_not_redeclared	Inherited namespaces not re-emitted on child elements
attribute_only_xmlns_is_preserved	Xmlns via .attribute() only (no .namespace()) preserved

Verification

cargo test -p xsd-parser-types --features quick-xml --lib xml::element::tests

[Bergmann89]

Hey @rcbevans,

Thanks for providing this fix. I have limited time this week, so it may take me until the weekend to complete the review.