Fix Multipart over HTTP, close AsyncHttpClient#1013

Author: slandelle

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

@@ -76,12 +76,12 @@ public byte[] getBoundary() {
     // RandomAccessBody API, suited for HTTP but not for HTTPS
     public long transferTo(long position, WritableByteChannel target) throws IOException {
 
-        long overallLength = 0;
-
         if (transfertDone) {
-            return contentLength;
+            return -1;
         }
 
+        long overallLength = 0;
+
         for (Part part : parts) {
             overallLength += part.write(target, boundary);
         }