fix(gridfs): make a copy of chunk before writing to server

A shared buffer is used for gridfs uploads, but there is a race
where the buffer could be changed before the write has a chance to
execute. Instead, we cut a copy of the buffer for use during the
write operation.

Author: mbroadst

lib/gridfs-stream/upload.js

@@ -422,7 +422,7 @@ function doWrite(_this, chunk, encoding, callback) {
       if (_this.md5) {
         _this.md5.update(_this.bufToStore);
       }
-      var doc = createChunkDoc(_this.id, _this.n, _this.bufToStore);
+      var doc = createChunkDoc(_this.id, _this.n, Buffer.from(_this.bufToStore));
       ++_this.state.outstandingRequests;
       ++outstandingRequests;