RespondAsync doesn't mention anyone

Created by: VelvetThePanda

Summary

Calling CommandContext#RespondAsync, DiscordMessage#RespondAsync, or even using a MessageBuilder and supplying true as the second parameter seems to completely ignore passed mentions, and will not mention any role/user.

Details

This seems to be in part due to some change in the ApiClient, where mentions aren't being passed to the REST Payload. DiscordChannel#SendMessageAsync doesn't have this issue as it uses a different overload that does seem to pass mentions.

Though at the same time this could be a DAPI change? I'm not too sure.

Steps to reproduce

• Get a channel.
• Get a message object (or message builder).
• Create a message that mentions someone.
• Send that message.
• Observe.

Notes

This could be intentional design, but I feel like we should allow all mentions by default.