Recommended fix for CORs immutable headers issue

[jacwright]

Fixes #261 by creating a new response with mutable headers and avoiding duplicating the body. The correct fix for what 74f63ab#diff-a1624121bc1fee18f8a5c2b2f907f01ba9c56a50068007e4c5da2911e91ea6df was trying to achieve.

This doesn't have changelog or version bumps. Just wanted to share how the code should run. I have it successfully running in my project.

[casertap]

In my opinion, this is an absolute must have @kwhitley
Thank you so much @jacwright for this excellent work.
I discovered this issue through wrangler flagging that with this ERROR.

[alexkli]

Thanks! Ran into TypeError: Can't modify immutable headers. in a Cloudflare worker when using corsify and sending a redirect using Response.redirect(). This change fixes it. Would be great to merge!