Sanely handle optional fields set to their default values.

Specify when this is allowed and when it's prohibited.

src/cpp/ripple/FieldNames.h

@@ -30,7 +30,8 @@ enum SOE_Flags
 {
 	SOE_INVALID  = -1,
 	SOE_REQUIRED = 0,	// required
-	SOE_OPTIONAL = 1,	// optional
+	SOE_OPTIONAL = 1,	// optional, may be present with default value
+	SOE_DEFAULT  = 2,   // optional, if present, must not have default value
 };
 
 class SField