SAP module sap_soap_rfc_eps_get_directory_listing cleanup

Hi @nmonkee,

This pull requests tries to cleanup https://github.com/rapid7/metasploit-framework/pull/1661. It's applying the same changes proposed on last pull requests such as https://github.com/rapid7/metasploit-framework/pull/1658. As always feel free to review, test, ask, discuss anything and merge this pull request on your repo when ready. So the original pull request will be automatically updated and we'll be ready to merge :)

Testing after changes: