Last image is not removed from initialPreviewConfig after delete

[koxu1996]

While trying to fix #779 I ran into this issue.

Reproduce:

1. Go to: http://plugins.krajee.com/file-preview-management-demo#preview-data
2. Delete all images
3. See initialPreviewConfig by entering this in web console: JSON.stringify($('#input-pd').data('fileinput').initialPreviewConfig)
4. Now you can see there is one element in initialPreviewConfig left, for example: "[null,null,{"type":"video","size":375000,"filetype":"video/mp4","caption":"KrajeeSample.mp4","url":"/file-upload-batch/2","key":3},null,null,null]", although fileinput is empty!

What cause this:
https://github.com/kartik-v/bootstrap-fileinput/blob/master/js/fileinput.js#L231-L233
Like in this question http://stackoverflow.com/questions/40317226/two-references-to-object-but-when-modify-it-by-one-second-still-show-unchanged, here we have initialPreviewConfig referenced by previewCache.data[id].config, so writing previewCache.data[id].config = [] would not change initialPreviewConfig.