[webservices] Extensions can be unknown XmlElements also, fix.

[webservices] Extensions can be unknown XmlElements also, fix.

* class/System.Web.Services/System.Web.Services.Description/BasicProfileChecker.cs:
Extensions can be XmlElement or ServiceDescriptionFormatExtension.
* class/System.Web.Services/System.Web.Services.Description/ServiceDescription.cs:
(AddExtensionNamespaces): Likewise. Also, ignore namespaces from
XmlElement, as we retain and write them out as-is.
(WriteExtension): Emit the XmlElement as-is.
* class/System.Web.Services/Test/System.Web.Services.Description/ServiceDescriptionTest.cs:
Update to write out the wsdl also.