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.txt

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