use batchLoadFn.apply instead of directly invoking batchLoadFn

use batchLoadFn.apply(loader,[keys]) instead of batchLoadFn(keys) to allow acces to the actual dataloader. Needed for example to use dataloader.prime() in the batch loader function.

src/index.js

@@ -253,7 +253,7 @@ function dispatchQueueBatch<K, V>(
 
   // Call the provided batchLoadFn for this loader with the loader queue's keys.
   var batchLoadFn = loader._batchLoadFn;
-  var batchPromise = batchLoadFn(keys);
+  var batchPromise = batchLoadFn.apply(loader, [ keys ]);
 
   // Assert the expected response from batchLoadFn
   if (!batchPromise || typeof batchPromise.then !== 'function') {