Add warning when deleting default middleware

[picsoung]

Today I faced some issues after deleting the default middleware and adding my own.

It looks like I could do use compressed JSON anymore...

@kikito found this.

It would be great to add a warning either:

• when you delete the default middleware
• when you use json.decode and you get the typical error

[mikz]

I would rather make the gzip transparent to the user. Closing in favour of #1.