Add overload for TIdHTTP.Delete() to send body data

[rlebeau]

Some REST APIs, like Spotify, use DELETE commands that require a request body:

https://developer.spotify.com/console/delete-playlist-tracks/

To make this easier to work with, TIdHTTP should expose TStrings and TStream overloads of the Delete() method to call DoRequest() with body data.

[rlebeau]

Consider adding to TIdHTTPHelper for #254, and then roll it into TIdHTTP later.