INT-4547: (S)FTP RFOG MPUT with collection payload

JIRA: https://jira.spring.io/browse/INT-4547

* PR Comments; use `MutableMessage` internally.

src/reference/asciidoc/ftp.adoc

@@ -1172,7 +1172,8 @@ The `mput` sends multiple files to the server and supports only one option:
 * `-R`: Recursive.
 Send all files (possibly filtered) in the directory and its subdirectories.
 
-The message payload must be a `java.io.File` that represents a local directory.
+The message payload must be a `java.io.File` (or `String`) that represents a local directory.
+Since version 5.1, a collection of `File` or `String` is also supported.
 
 This command supports the same attributes as the <<ftp-put-command,`put` command>>.
 In addition, files in the local directory can be filtered with one of `mput-pattern`, `mput-regex`, `mput-filter`, or `mput-filter-expression`.

src/reference/asciidoc/sftp.adoc

@@ -1136,7 +1136,8 @@ When configuring the adapter using java, you can use `setChmod(0600)`.
 
 * `-R`: Recursive -- send all files (possibly filtered) in the directory and subdirectories
 
-The message payload must be a `java.io.File` that represents a local directory.
+The message payload must be a `java.io.File` (or `String`) that represents a local directory.
+Since version 5.1, a collection of `File` or `String` is also supported.
 
 The same attributes as the <<sftp-put-command,`put` command>> are supported.
 In addition, you can filter files in the local directory with one of `mput-pattern`, `mput-regex`, `mput-filter`, or `mput-filter-expression`.

src/reference/asciidoc/whats-new.adoc

@@ -166,6 +166,9 @@ The `CachingSessionFactory` has a new property `testSession` which, when true, c
 
 See <<sftp-session-caching>> and <<ftp-session-caching>> for more information.
 
+The outbound gateway MPUT command now supports a message payload with a collection of files or strings.
+See <<sftp-outbound-gateway>> and <<ftp-outbound-gateway>> for more information.
+
 [[x51.-tcp]]
 === TCP Support