support for control encoding

[pashtetkun]

Description:

please, add support for possibility to set control encoding for ftpClient (for method isFileExist and etc).
See info - https://issues.apache.org/jira/browse/VFS-302 -i.e. set optional call for client.setControlEncoding("utf-8") before client.createConnection()

This support is useful for situation when the file pathes in ftp server are not ascii (for example - consist cyrillic and other symbols)

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