Introduce method-invoking handle

* Add generic argument to `GenericEndpointSpec`.
Since `EndpointSpec#get()` returns `Tuple` with `EndpointFactoryBean` and `MessageHandler` objects
it is useful to get deal with specific generic for further `MessageHandler` configuration within `EndpointConfigurer` lambda
* Change DSL-methods to get deal deal with those generics.
* Add `FileWritingMessageHandler` test to demonstrate it.
* Fix bug around double `MessageHandler` bean registration from `DslIntegrationConfigurationInitializer`, when handler is a reference
to existing bean.

spring-integration-java-dsl/build.gradle

@@ -58,6 +58,7 @@ dependencies {
 	testCompile "org.springframework:spring-test:$springVersion"
 
 	testCompile "org.springframework.integration:spring-integration-event:$springIntegrationVersion"
+	testCompile "org.springframework.integration:spring-integration-file:$springIntegrationVersion"
 
 	jacoco group: "org.jacoco", name: "org.jacoco.agent", version: "0.5.6.201201232323", classifier: "runtime"
 }