INT-4531: Use Object type KeyDirectory.key

JIRA: https://jira.spring.io/browse/INT-4531

The `DelegatingSessionFactory` uses plain `Object` as key type
whereas the default `RotatingServerAdvice` does not.

* Change the type from `String` to `Object` in the `RotatingServerAdvice.KeyDirectory`
* Fix typos in the (s)ftp.adoc

**Cherry-pick to 5.0.x**

spring-integration-file/src/main/java/org/springframework/integration/file/remote/aop/RotatingServerAdvice.java

@@ -35,6 +35,7 @@ import org.springframework.util.Assert;
  * A smart poller advice that rotates across multiple remote servers/directories.
  *
  * @author Gary Russell
+ * @author Michael Forstner
  *
  * @since 5.0.7
  *
@@ -196,9 +197,9 @@ public class RotatingServerAdvice extends AbstractMessageSourceAdvice {
 
 		protected void configureSource(MessageSource<?> source) {
 			Assert.isTrue(source instanceof AbstractInboundFileSynchronizingMessageSource
-					|| source instanceof AbstractRemoteFileStreamingMessageSource,
-				"source must be an AbstractInboundFileSynchronizingMessageSource or a "
-				+ "AbstractRemoteFileStreamingMessageSource");
+							|| source instanceof AbstractRemoteFileStreamingMessageSource,
+					"source must be an AbstractInboundFileSynchronizingMessageSource or a "
+							+ "AbstractRemoteFileStreamingMessageSource");
 			if (!this.iterator.hasNext()) {
 				this.iterator = this.keyDirectories.iterator();
 			}
@@ -208,7 +209,7 @@ public class RotatingServerAdvice extends AbstractMessageSourceAdvice {
 			}
 			else {
 				((AbstractInboundFileSynchronizingMessageSource<?>) source).getSynchronizer()
-					.setRemoteDirectory(this.current.getDirectory());
+						.setRemoteDirectory(this.current.getDirectory());
 			}
 		}
 
@@ -219,18 +220,18 @@ public class RotatingServerAdvice extends AbstractMessageSourceAdvice {
 	 */
 	public static class KeyDirectory {
 
-		private final String key;
+		private final Object key;
 
 		private final String directory;
 
-		public KeyDirectory(String key, String directory) {
+		public KeyDirectory(Object key, String directory) {
 			Assert.notNull(key, "key cannot be null");
 			Assert.notNull(directory, "directory cannot be null");
 			this.key = key;
 			this.directory = directory;
 		}
 
-		public String getKey() {
+		public Object getKey() {
 			return this.key;
 		}
 
@@ -240,7 +241,7 @@ public class RotatingServerAdvice extends AbstractMessageSourceAdvice {
 
 		@Override
 		public String toString() {
-			return "KeyDirectory [key=" + this.key + ", directory=" + this.directory + "]";
+			return "KeyDirectory [key=" + this.key.toString() + ", directory=" + this.directory + "]";
 		}
 
 	}

src/reference/asciidoc/ftp.adoc

@@ -247,7 +247,7 @@ The following example shows how to declare a delegating session factory:
 IMPORTANT: When you use session caching (see <<ftp-session-caching>>), each of the delegates should be cached.
 You cannot cache the `DelegatingSessionFactory` itself.
 
-Starting with _version 5.0.7_, the `DelegatingSessionFactory` can be used in conjuction with a `RotatingServerAdvice` to poll multiple servers; see <<ftp-rotating-server-advice>>.
+Starting with _version 5.0.7_, the `DelegatingSessionFactory` can be used in conjunction with a `RotatingServerAdvice` to poll multiple servers; see <<ftp-rotating-server-advice>>.
 
 [[ftp-inbound]]
 === FTP Inbound Channel Adapter

src/reference/asciidoc/sftp.adoc

@@ -210,7 +210,7 @@ We added convenience methods so that you can more easily do so from a message fl
 IMPORTANT: When using session caching (see <<sftp-session-caching>>), each of the delegates should be cached.
 You cannot cache the `DelegatingSessionFactory` itself.
 
-Starting with _version 5.0.7_, the `DelegatingSessionFactory` can be used in conjuction with a `RotatingServerAdvice` to poll multiple servers; see <<sftp-rotating-server-advice>>.
+Starting with _version 5.0.7_, the `DelegatingSessionFactory` can be used in conjunction with a `RotatingServerAdvice` to poll multiple servers; see <<sftp-rotating-server-advice>>.
 
 [[sftp-session-caching]]
 === SFTP Session Caching