Fix issues with message conversion in mongodb sink

Fore more info see: https://stackoverflow.com/questions/67131500/mongodb-sink-issue-while-writing-a-json-data-from-http-source

Resolves https://github.com/spring-cloud/stream-applications/issues/165

This commit also addresses a doc update for sftp-source

applications/sink/mongodb-sink/pom.xml

@@ -53,14 +53,17 @@
                         <version>${project.version}</version>
                         <configClass>org.springframework.cloud.fn.consumer.mongo.MongoDbConsumerConfiguration.class
                         </configClass>
-                        <functionDefinition>mongodbConsumer</functionDefinition>
-
+                        <functionDefinition>byteArrayTextToString|mongodbConsumer</functionDefinition>
                         <maven>
                             <dependencies>
                                 <dependency>
                                     <groupId>org.springframework.cloud.fn</groupId>
                                     <artifactId>mongodb-consumer</artifactId>
                                 </dependency>
+                                <dependency>
+                                    <groupId>org.springframework.cloud.fn</groupId>
+                                    <artifactId>payload-converter-function</artifactId>
+                                </dependency>
                             </dependencies>
                         </maven>
                     </application>

applications/source/sftp-source/README.adoc

@@ -144,6 +144,7 @@ $$max-fetch$$:: $$The maximum number of remote files to fetch per poll; default
 $$preserve-timestamp$$:: $$Set to true to preserve the original timestamp.$$ *($$Boolean$$, default: `$$true$$`)*
 $$remote-dir$$:: $$The remote FTP directory.$$ *($$String$$, default: `$$/$$`)*
 $$remote-file-separator$$:: $$The remote file separator.$$ *($$String$$, default: `$$/$$`)*
+$$rename-remote-files-to$$:: $$A SpEL expression resolving to the new name remote files must be renamed to after successful transfer.$$ *($$Expression$$, default: `$$<none>$$`)*
 $$stream$$:: $$Set to true to stream the file rather than copy to a local directory.$$ *($$Boolean$$, default: `$$false$$`)*
 $$tmp-file-suffix$$:: $$The suffix to use while the transfer is in progress.$$ *($$String$$, default: `$$.tmp$$`)*