fix: [274]: Requested URL is incorrect when PostHog host includes a path

[bcheidemann]

💡 Motivation and Context

Fixes PostHog#274
Fixes PostHog/posthog-flutter#128

💚 How did you test it?

1. Add unit tests asserting the correct behaviour and asserting preservation of existing related behaviour
2. Run make test to ensure all tests passing
3. Run make build to ensure the build is passing

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed. (I believe this is not needed)
• No breaking change or entry added to the changelog. (assuming from the wording that I should not update the changelog)

[bcheidemann]

@marandaneto @ioannisj please let me know if you'd like any changes made to this PR 🙂 I've tried to follow the conventions in your codebase as far as possible so hopefully I'm not too far off! However, this is my first time writing Swift, so I'd especially value any feedback you have for me - particularly around writing idiomatic Swift code.

Also, I'm assuming the new version of posthog-ios needs to be released before I can update posthog-flutter?