INT-3516: Allow Optional<> in POJO Method Args

JIRA: https://jira.spring.io/browse/INT-3516

* Add `Optional<>` support for `@Header` in the `MessagingMethodInvokerHelper`
* Add `Optional<>` test-case
* Change `sourceCompatibility` for test to the Java 8

INT-3516: Revert SF version to 4.1.1

Add Docs on the matter

Fix WS Tests; Revert StubJavaMailSender

src/reference/docbook/service-activator.xml

@@ -53,7 +53,26 @@
        not worrying about the contents of the message. Think of it as a NULL JMS message. An example use-case for such an
        implementation could be a simple counter/monitor of messages deposited on the input channel.
     </note>
-    <para>
+      <para>
+          Starting with <emphasis>version 4.1</emphasis> the framework correct converts Message properties
+          (<code>payload</code> and <code>headers</code>) to the Java 8 <classname>Optional</classname> POJO method
+          parameters:
+          <programlisting language="java"><![CDATA[public class MyBean {
+    public String computeValue(Optional<String> payload,
+               @Header(value="foo", required=false) String foo1,
+               @Header(value="foo") Optional<String> foo2) {
+        if (payload.isPresent()) {
+            String value = payload.get();
+            ...
+        }
+        else {
+           ...
+       }
+    }
+
+}]]></programlisting>
+      </para>
+      <para>
       Using a "ref" attribute is generally recommended if the custom Service Activator handler implementation can be reused
       in other <code>&lt;service-activator&gt;</code> definitions. However if the custom Service Activator handler implementation
       is only used within a single definition of the <code>&lt;service-activator&gt;</code>, you can provide an inner bean definition:

src/reference/docbook/whats-new.xml

@@ -230,5 +230,12 @@
 				See <xref linkend="resequencer"/>.
 			</para>
 		</section>
+		<section id="4.1-Optional-Parameter">
+			<title>Optional POJO method parameter</title>
+			<para>
+				Now Spring Integration consistently handles the Java 8's <classname>Optional</classname> type.
+				See <xref linkend="service-activator-namespace"/>.
+			</para>
+		</section>
 	</section>
 </chapter>