Minimize readResolve visibility (#2567)

* Minimize readResolve visibility

* Relax visibility from private to protected

core/src/main/java/hudson/ProxyConfiguration.java

@@ -204,7 +204,7 @@ public final class ProxyConfiguration extends AbstractDescribableImpl<ProxyConfi
         SaveableListener.fireOnChange(this, config);
     }
 
-    public Object readResolve() {
+    private Object readResolve() {
         if (secretPassword == null)
             // backward compatibility : get scrambled password and store it encrypted
             secretPassword = Secret.fromString(Scrambler.descramble(password));