Fixed integer overflow when file size > Integer.MAX_INT, close AsyncHttpClient#1342

Motivation:

`FilePart` currently uses an int to track written length. Huge files can cause an overflow.

Modification:

Use a long instead of an int

Result:
No more overflow

Author: gculik

src/main/java/com/ning/http/client/multipart/FilePart.java

@@ -101,7 +101,7 @@ public long write(WritableByteChannel target, byte[] boundary) throws IOExceptio
 
         handler.start();
 
-        int length = 0;
+        long length = 0;
 
         length += MultipartUtils.writeBytesToChannel(target, generateFileStart(boundary));