INT-4303: (S)FTP MGET with FileExistsMode.IGNORE

JIRA: https://jira.spring.io/browse/INT-4303

Don't emit ignored files in the output message payload.

Also, always preserve the timestamp when using `FileExistsMode.REPLACE_IF_MODIFIED`.

Some code style polishing

spring-integration-ftp/src/test/java/org/springframework/integration/ftp/outbound/FtpServerOutboundTests-context.xml

@@ -42,6 +42,7 @@
 							  command="mget"
 							  command-options="-f -P"
 							  expression="payload"
+							  mode="IGNORE"
 							  local-directory-expression="@extraConfig.targetLocalDirectoryName + (#remoteDirectory ?: '')"
 							  local-filename-generator-expression="#remoteFileName.replaceFirst('ftpSource', 'localTarget')"
 							  reply-channel="output"/>

spring-integration-ftp/src/test/java/org/springframework/integration/ftp/outbound/FtpServerOutboundTests.java

@@ -228,6 +228,11 @@ public class FtpServerOutboundTests extends FtpTestSupport {
 		for (File file : localFiles) {
 			assertThat(file.getPath().replaceAll(Matcher.quoteReplacement(File.separator), "/"), containsString(dir));
 		}
+		this.inboundMGet.send(new GenericMessage<Object>(dir + "*.txt"));
+		result = this.output.receive(1000);
+		assertNotNull(result);
+		localFiles = (List<File>) result.getPayload();
+		assertThat(localFiles.size(), equalTo(0));
 	}
 
 	@Test