Incorrectly generated code for `mixed="true"` complex types about xgen

[halostatue]

Note that GoComply/xsd2go generates this correctly. If xgen were to generate this correctly, there would be another issue for xsd:extends where the parent type has mixed="true" and the child type has mixed="false" or unspecified. Specifically, xgen makes the choice to generate the following code for such an entry:

// SharedTypeCustomAttributes ...
type SharedTypeCustomAttributes struct {
	XMLName         xml.Name                     `xml:"sharedType.CustomAttributes"`
	Customattribute []*SharedTypeCustomAttribute `xml:"custom-attribute"`
}

// SharedTypeCustomAttribute ...
type SharedTypeCustomAttribute struct {
	XMLName         xml.Name `xml:"sharedType.CustomAttribute"`
	AttributeidAttr string   `xml:"attribute-id,attr"`
	XmlLangAttr     string   `xml:"xml:lang,attr,omitempty"`
	Value           []string `xml:"value"`
}

// SharedTypeSiteSpecificCustomAttributes ...
type SharedTypeSiteSpecificCustomAttributes struct {
	XMLName         xml.Name                                 `xml:"sharedType.SiteSpecificCustomAttributes"`
	Customattribute []*SharedTypeSiteSpecificCustomAttribute `xml:"custom-attribute"`
}

// SharedTypeSiteSpecificCustomAttribute ...
type SharedTypeSiteSpecificCustomAttribute struct {
	XMLName    xml.Name `xml:"sharedType.SiteSpecificCustomAttribute"`
	SiteidAttr string   `xml:"site-id,attr,omitempty"`
	*SharedTypeCustomAttribute
}

If SharedTypeCustomAttribute gets InnerXml generated, then SharedTypeSiteSpecificCustomAttribute will also implicitly get InnerXml through the embedding. This is something not well defined in the XSD specification, but that discussions from 2005 on the XSD mailing list and 2012 on StackOverflow strongly suggest that the mixed attribute is not inherited. That means that SharedTypeSiteSpecificCustomAttribute would need to be implemented separately and not via embedded inheritance since the mixed attribute differs (or, more specifically, that mixed=false is stricter than mixed=true). (The optimization could be kept if the mixed attributes matched, and I believe that there is a bug in the SFCC impex XSD which I have reported into the SFCC black hole.)