Several proxy request and cache fixes for the fonts and 3rd-parties examples

[pmeenan]

• Filter the response headers for proxied requests.
• Always specify a UA string when fetching font CSS.
• Used the default cache (instead of a named cache).
• Tell the worker runtime to wait for the cache puts.
• Fixed a few (non-critical) missing semicolons.

[pmeenan]

@zackbloom - Hopefully the last PR for a while (sorry for the churn). Last tweaks were based on feedback from the Workers team.