Fix FilePart position when using zero-copy, close AsyncHttpClient#1123

Author: slandelle

client/src/main/java/org/asynchttpclient/request/body/multipart/part/FileMultipartPart.java

@@ -59,7 +59,9 @@ protected long transferContentTo(ByteBuf target) throws IOException {
 
     @Override
     protected long transferContentTo(WritableByteChannel target) throws IOException {
-        long transferred = channel.transferTo(channel.position(), BodyChunkedInput.DEFAULT_CHUNK_SIZE, target);
+        // WARN: don't use channel.position(), it's always 0 here
+        // from FileChannel javadoc: "This method does not modify this channel's position."
+        long transferred = channel.transferTo(position, BodyChunkedInput.DEFAULT_CHUNK_SIZE, target);
         position += transferred;
         if (position == length) {
             state = MultipartState.POST_CONTENT;