MultipartBody#read() returns -1 when it meets EOF.

Current MultipartBody#read() returns only 0 when it meets both Exception and EOF(endWritten). Then caller can't know whether the value is EOF or
Error.

Author: carryel

api/src/main/java/org/asynchttpclient/multipart/MultipartBody.java

@@ -81,7 +81,7 @@ public long read(ByteBuffer buffer) throws IOException {
             int maxLength = buffer.remaining();
 
             if (startPart == parts.size() && endWritten) {
-                return overallLength;
+                return -1;
             }
 
             boolean full = false;