fix(TextBasedChannel): allow passing an APIMessage with split

[almeidx]

Please describe the changes this PR makes and why it should be merged:
Previously, replying to a message (or passing an APIMessage to a TextBasedChannel#send) with split enabled threw a DiscordAPIError: Invalid Form Body.

Example:

message.reply({ content: '<message content>', split: true });

Error:

DiscordAPIError: Invalid Form Body
content: Could not interpret "['```\n<message content>\n```']" as string.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

[SpaceEEC]

There is another occurrence of this pattern in webhook, can you include that fix there too?

discord.js/src/structures/Webhook.js

Lines 157 to 164 in e3e466d

	if (options instanceof APIMessage) {
	apiMessage = options.resolveData();
	} else {
	apiMessage = APIMessage.create(this, options).resolveData();
	if (Array.isArray(apiMessage.data.content)) {
	return Promise.all(apiMessage.split().map(this.send.bind(this)));
	}
	}