encoding/xml: Unmarshal does not properly handle NCName in XML namespaces

Example:
type A struct {
  XMLName xml.Name `xml:"a:B"`
}
Struct A is marshaled to "<a:B></a:B>", as expected.
However if we run 
a := A{}
xml.Unmarshal([]byte("<a:B></a:B>"), &a)

It results in an error saying "unexpected error: expected element type <a:B> but have <B>"
