Adding assertion for empty data in -appendPartWithHeaders:body:

AFNetworking/AFHTTPClient.m

@@ -851,6 +851,8 @@ NSTimeInterval const kAFUploadStream3GSuggestedDelay = 0.2;
 - (void)appendPartWithHeaders:(NSDictionary *)headers
                          body:(NSData *)body
 {
+    NSParameterAssert(body);
+    
     AFHTTPBodyPart *bodyPart = [[AFHTTPBodyPart alloc] init];
     bodyPart.stringEncoding = self.stringEncoding;
     bodyPart.headers = headers;