Fix ArrayIndexOutOfBounds when writing big buffer

If ChannelOutputStream.write(byte[], int, int) was called with a buffer larger 
than bufferSize the loop in that method would call DataBuffer.write with a small len
and a large off.  This would cause the calculation in line 90 to return a negative n
leading to a ArrayIndexOutOfBounds.  The offset should not be taken into account when
calculating the number of bytes to put in the buffer.
Merge pull request #104 from mpoindexter/master

ArrayIndexOutOfBounds when writing to a SFTP RemoteFile's OutputStream with large buffer