SFTP: Add support for "email@example.com" extension for SFTP client. #471
this is the same feature as #65, but with docstrings and a test.
Note that at the moment, the test fails early if run against the sftp stub server, as the stub server actually uses os.rename() to implement the sftp rename, which is wrong as per the sftp spec. However, the test works if run against openssh.