xml: match Marshal's XMLName behavior in Unmarshal

When xml.Marshal is called on a struct it will happily
reflect the information in the "tag" of an XMLName member
regardless of the type to give the struct a tag-name in
it's XML form. This is backed up by the documentation which
says:

However xml.Unmarshal *does* care about the XMLName field
being of type xml.Name, and currently returns the error
"field XMLName does not have type xml.Name" if you have it
set to something else.

This is firstly inconsistant with xml.Marshal but it also
makes it impossible to use xml.Marshal alongside other
Marshallers (like json/bson) without poluting the state's
namespace with XMLName fields. Inorder to exclude fields
from other Marshallers the convention has been started to
tag fields as "omitempty"; which will cause the field not
to display if it is at it's "zero" state, XMLName cannot
have such as zero-state since it is a struct, so it is nicer
to use a pointer/bool value for XMLName so it can be easily
excluded when I want to Marshal my struct by some other
wire format.

Attached is the proposed minor change, that simply stops
erring if it can't set the name on the XMLName field, which
is just optional metadata anyway.
Fixes #2265.

R=rsc
CC=golang-dev
https://golang.org/cl/5067044

src/pkg/xml/read.go

@@ -321,10 +321,9 @@ func (p *Parser) unmarshal(val reflect.Value, start *StartElement) os.Error {
 
 			// Save
 			v := sv.FieldByIndex(f.Index)
-			if _, ok := v.Interface().(Name); !ok {
-				return UnmarshalError(sv.Type().String() + " field XMLName does not have type xml.Name")
+			if _, ok := v.Interface().(Name); ok {
+				v.Set(reflect.ValueOf(start.Name))
 			}
-			v.Set(reflect.ValueOf(start.Name))
 		}
 
 		// Assign attributes.

src/pkg/xml/read_test.go

@@ -369,3 +369,25 @@ var attrStruct = AttrTest{
 		Bool: true,
 	},
 }
+
+// test data for TestUnmarshalWithoutNameType
+
+const OK = "OK"
+const withoutNameTypeData = `
+<?xml version="1.0" charset="utf-8"?>
+<Test3 attr="OK" />`
+
+type TestThree struct {
+	XMLName bool   `xml:"Test3"` // XMLName field without an xml.Name type 
+	Attr    string `xml:"attr"`
+}
+
+func TestUnmarshalWithoutNameType(t *testing.T) {
+	var x TestThree
+	if err := Unmarshal(StringReader(withoutNameTypeData), &x); err != nil {
+		t.Fatalf("Unmarshal: %s", err)
+	}
+	if x.Attr != OK {
+		t.Fatalf("have %v\nwant %v", x.Attr, OK)
+	}
+}