Is your feature request related to a problem? Please describe.

Already discussed in #3658. Quote:

Slim framework doesn't parse request body in GET and DELETE methods which seems fair. However you still can get raw body data(param1=foo&param2=bar when Content-Type: application/x-www-form-urlencoded

Describe the solution you'd like

I think we should throw a warning when GET method contains parameter in body. I'll implement it myself, this issue just to track progress.