[http] Fix some routing issues
[http] Fix some routing issues

Fix the following issues with the routing:

- AddMemberRole now POSTs instead of DELETEs
- GuildBanUser now POSTs instead of DELETEs
- CreateRole now POSTs instead of DELETEs
- CreateWebhook now POSTs instead of DELETEs

These issues arose from commit hash bbbf63868a8ef3c0f21c1896f7afb96f4d8fbcc1.

Lakelezz/serenity,zeyla/serenity,Flat/serenity,zeyla/serenity.rs,acdenisSK/serenity