MULE-7821 set SftpException as cause when re-throwing IOException in listFiles
Merge branch 'mule-3.4.x' of github.com:Confluex/mule into mule-3.4.x

Conflicts:
	transports/sftp/src/main/java/org/mule/transport/sftp/SftpClient.java
	transports/sftp/src/test/java/org/mule/transport/sftp/SftpClientTestCase.java