Allow underscores in XML element names (except for leading characters)

Fixes #569

R=rsc, r
CC=golang-dev
https://golang.org/cl/194121

src/pkg/xml/xml_test.go

@@ -5,6 +5,7 @@
 package xml
 
 import (
+	"bytes"
 	"io"
 	"os"
 	"reflect"
@@ -212,3 +213,18 @@ func TestSyntax(t *testing.T) {
 		}
 	}
 }
+
+type item struct {
+	Field_a string
+}
+
+func TestIssue569(t *testing.T) {
+	data := `<item><field_a>abcd</field_a></item>`
+	var i item
+	buf := bytes.NewBufferString(data)
+	err := Unmarshal(buf, &i)
+
+	if err != nil || i.Field_a != "abcd" {
+		t.Fatalf("Expecting abcd")
+	}
+}