fix(ApiMessage): respect allowedMentions with split

[almostSouji]

Please describe the changes this PR makes and why it should be merged:

• ⚠closes using message split ignores message options #4300

An oversight in two positions leads to allowedMentions being ignored when splitOptions are present.

Breakdown of changes

• https://github.com/discordjs/discord.js/pull/4588/files#diff-3916ea503ae839d34e6dd447ef035977R194 : required to not pass undefined if content is an array (which is the case when split options are passed)
• https://github.com/discordjs/discord.js/pull/4588/files#diff-3916ea503ae839d34e6dd447ef035977R249-R250 : allowed_mentions/allowedMentions need to be passed to split messages that are not the last message as well. allowed_mentions is used in data since it represents the data sent to the API. allowedMentions is used in opt since it represents APIMessage#options and thus needs to follow library spec.

Status

• 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

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.