[email-plugin] Filters don't work when loadData used

[michaelbromley]

Describe the bug
When defining an EmailEventHandler, if the loadData method is used after the filter method, then the filters no longer work.

To Reproduce

const handler = new EmailEventListener('test')
                .on(MockEvent)
                .filter(event => event.shouldSend === true)
                .loadData(context => Promise.resolve('loaded data'))
                .setRecipient(() => '[email protected]')
                .setFrom('"test from" <[email protected]>')
                .setSubject('test subject');

Expected behavior
Filters should still work

Environment (please complete the following information):

• @vendure/core version: 0.8.1
• Nodejs version: 12.4.0
• Database (mysql/postgres etc): any

Additional context
The cause seems to be due to this:

vendure/packages/email-plugin/src/event-handler.ts

Lines 149 to 153 in 1cd27a1

	loadData<R>(
	loadDataFn: LoadDataFn<Event, R>,
	): EmailEventHandlerWithAsyncData<R, T, Event, EventWithAsyncData<Event, R>> {
	return new EmailEventHandlerWithAsyncData(loadDataFn, this.listener, this.event);
	}

When the EmailEventHandlerWithAsyncData is created, the this.filterFns array is not passed on.