reflect: don't construct an interface-in-interface value

v.Interaface() could construct an interface in interface value if v was
of type interface. This is not correct, and doesn't follow upstream Go
behavior. Instead, it should return the interface value itself.

testdata/reflect.go

@@ -321,6 +321,9 @@ func main() {
 
 	println("\nstruct tags")
 	TestStructTag()
+
+	println("\nv.Interface() method")
+	testInterfaceMethod()
 }
 
 func emptyFunc() {
@@ -470,6 +473,19 @@ func TestStructTag() {
 	println(field.Tag.Get("color"), field.Tag.Get("species"))
 }
 
+// Test Interface() call: it should never return an interface itself.
+func testInterfaceMethod() {
+	v := reflect.ValueOf(struct{ X interface{} }{X: 5})
+	println("kind:", v.Field(0).Kind().String())
+	itf := v.Field(0).Interface()
+	switch n := itf.(type) {
+	case int:
+		println("int", n) // correct
+	default:
+		println("something else") // incorrect
+	}
+}
+
 var xorshift32State uint32 = 1
 
 func xorshift32(x uint32) uint32 {

testdata/reflect.txt

@@ -377,3 +377,7 @@ type assertion succeeded for unreferenced type
 
 struct tags
 blue gopher
+
+v.Interface() method
+kind: interface
+int 5