fix: Send beacon request encoding

[benjackwhite]

Changes

I was suspicious that sendBeacon was broken (it isn't) but either way it behaved slightly different to before and needed tests.
Validated it worked in the playground app locally.

Checklist

• Tests for new code (see advice on the tests we use)
• Accounted for the impact of any changes across different browsers
• Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

[github-actions]

Size Change: -76 B (0%)

Total Size: 852 kB

Filename	Size	Change
dist/array.full.js	182 kB	-24 B (0%)
dist/array.js	124 kB	-23 B (0%)
dist/es.js	124 kB	-23 B (0%)
dist/module.js	124 kB	-6 B (0%)

ℹ️ View Unchanged

Filename	Size
dist/exception-autocapture.js	12.1 kB
dist/recorder-v2.js	106 kB
dist/recorder.js	58.7 kB
dist/surveys-module-previews.js	62.1 kB
dist/surveys.js	58.4 kB

_{compressed-size-action}

[pauldambra]

🚢