[SSHD-326] Fix for bug in DefaultSftpClient#write(String)

Previously, DefaultSftpClient#write(String) returned an OutputStream that had a
bug in OutputStream#write(byte[],int,int): Since off was never updated, anything
copied into the buffer was from the first 32 KiB of the array passed as first
argument.

This commit also makes sure that the OutputStream returned by
DefaultSftpClient#write(String) performs a flush on close().