Fetch Fixes

[nicoburniske]

Closes #10, #19

Fix GET and POST request bugs:

• GET: Remove lock check in code path where body is previously locked to resolve body disturbed error
• POST: Cache WASI HTTP headers in HttpOutgoingRequest and close body in write_all case

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Note: Async body in POST requests is not supported. This means when you make a fetch request and pass the response body directly into another fetch request without awaiting it first, like:

const response1 = await fetch(url1);
const response2 = await fetch(url2, { method: 'POST', body: response1.body });

This limitation should be documented and addressed separately from the fixes in this PR.