quick fix to an oopsie

Author: Will Banfield

src/GridFS/GridFsUpload.php

@@ -121,6 +121,7 @@ public function insertChunks($toWrite)
             $readBytes += strlen($addToBuffer);
             $this->bufferLength += strlen($addToBuffer);
             if($this->bufferLength == $this->chunkSizeBytes) {
+                rewind($this->buffer);
                 $this->insertChunk(fread($this->buffer, $this->chunkSizeBytes));
                 ftruncate($this->buffer,0);
                 $this->bufferLength = 0;