fix: dispatch must set [[waitForUpdate]] to true

[marcoscaceres]

closes #854

The following tasks have been completed:

• [x] Confirmed there are no ReSpec errors/warnings.
• [ ] Modified Web platform tests

Implementation commitment:

• [ ] Safari (link to issue)
• [ ] Chrome (link to issue)
• [x] Firefox - already implements this.
• [ ] Edge (public signal)

Optional, impact on Payment Handler spec?

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