Fix sending multipart bodies through SocketChannel.

As per documentation, selector.select() returns only keys that have
updated and therefore even if it return zero, it doesn't mean that
selectedKeys wouldn't contain keys that are writable.

See http://stackoverflow.com/questions/9939989/java-nio-selector-select-returns-0-although-channels-are-ready and http://docs.oracle.com/javase/7/docs/api/java/nio/channels/Selector.html#select() for more details.

This commit utilizes the same maxSpin already used in FileChannel case
for detecting that writing has stuck somewhere.  A configurable
timeout for select would be the obvious better solution.

Author: Petri Louhelainen

src/main/java/com/ning/http/multipart/MultipartBody.java

@@ -578,18 +578,21 @@ private long writeToTarget(WritableByteChannel target, ByteArrayOutputStream byt
                     final SocketChannel channel = (SocketChannel) target;
                     channel.register(selector, SelectionKey.OP_WRITE);
 
-                    while (written < byteWriter.size() && selector.select() != 0) {
+                    while (written < byteWriter.size()) {
+                        selector.select(1000);
+                        maxSpin++;
                         final Set<SelectionKey> selectedKeys = selector.selectedKeys();
 
                         for (SelectionKey key : selectedKeys) {
                             if (key.isWritable()) {
                                 written += target.write(message);
+                                maxSpin = 0;
                             }
                         }
-                    }
 
-                    if (written < byteWriter.size()) {
-                        throw new IOException("Unable to write on channel " + target);
+                        if (maxSpin >= 10) {
+                            throw new IOException("Unable to write on channel " + target);
+                        }
                     }
                 } finally {
                     selector.close();