Webhook DisplayName is lost through caching

Created by: spydacarnage

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Summary

When a call to DiscordChannel.GetMessagesAsync returns multiple messages created by the same Webhook, they are all attributed to the same user/member, with the same Username / DisplayName, even if each author object returned from the Discord API contains a different username.

Details

No exceptions are thrown, and this bug is operating system independent. I am running in .NET Core 3.1.

An example is the best way to describe this, I think: If the first message in the batch contains the following author property:

    "author": {
        "bot": true,
        "id": "123451234512345405",
        "username": "Bob",
        "avatar": null,
        "discriminator": "0000"
    },

and the next one contains:

    "author": {
        "bot": true,
        "id": "123451234512345405",
        "username": "Sarah",
        "avatar": null,
        "discriminator": "0000"
    },

both DiscordMessage objects in the returns List both have the same DiscordMember object, with the same Username and DisplayName (i.e. "Sarah").

The problem appears to be in DiscordApiClient.PrepareMessage, where the DiscordUser / DiscordMember is being retrieved from the cache based on the user's Id (which is always set to the Id of the webhook)

Notes

I've toyed around with adding a new WebhookName property to DiscordMember and then changing the DisplayName property to be this.WebhookName ?? this.Nickname ?? this.Username, and that appears to work, but I don't know if you have a better method of preserving this information than that.