fix(netlify): support middleware rewrite (#413)

* fix(netlify): support middleware rewrite

* Changeset

.changeset/fast-penguins-build.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/netlify': patch
+---
+
+Fixes `context.rewrite` in edge middleware

packages/netlify/src/index.ts

@@ -294,6 +294,25 @@ export default function netlifyIntegration(
 					params: {}
 				});
 				ctx.locals = { netlify: { context } }
+				// https://docs.netlify.com/edge-functions/api/#return-a-rewrite
+				ctx.rewrite = (target) => {
+					if(target instanceof Request) {
+						// We can only mutate headers, so if anything else is different, we need to fetch
+						// the target URL instead.
+						if(target.method !== request.method || target.body || target.url.origin !== request.url.origin) {
+							return fetch(target);
+						}
+						// We can't replace the headers object, so we need to delete all headers and set them again
+						request.headers.forEach((_value, key) => {
+							request.headers.delete(key);
+						});
+						target.headers.forEach((value, key) => {
+							request.headers.set(key, value);
+						});
+						return new URL(target.url);
+					}
+					return new URL(target, request.url);
+				};
 				const next = () => {
 					const { netlify, ...otherLocals } = ctx.locals;
 					request.headers.set("x-astro-locals", trySerializeLocals(otherLocals));