Revert "support Headers being passed in fetchOptions"

This reverts commit dc509cc.

Author: JoviDeCroock

.changeset/fluffy-poets-protect.md

@@ -107,30 +107,6 @@ describe('makeFetchOptions', () => {
     `);
   });
 
-  it('handles the Headers object', () => {
-    const headers = new Headers();
-    headers.append('x-test', 'true');
-    const operation = makeOperation(queryOperation.kind, queryOperation, {
-      ...queryOperation.context,
-      fetchOptions: {
-        headers,
-      },
-    });
-    const body = makeFetchBody(operation);
-
-    expect(makeFetchOptions(operation, body)).toMatchInlineSnapshot(`
-      {
-        "body": "{\\"operationName\\":\\"getUser\\",\\"query\\":\\"query getUser($name: String) {\\\\n  user(name: $name) {\\\\n    id\\\\n    firstName\\\\n    lastName\\\\n  }\\\\n}\\",\\"variables\\":{\\"name\\":\\"Clara\\"}}",
-        "headers": {
-          "accept": "application/graphql-response+json, application/graphql+json, application/json, text/event-stream, multipart/mixed",
-          "content-type": "application/json",
-          "x-test": "true",
-        },
-        "method": "POST",
-      }
-    `);
-  });
-
   it('creates a GET request when preferred for query operations', () => {
     const operation = makeOperation(queryOperation.kind, queryOperation, {
       ...queryOperation.context,

packages/core/src/internal/fetchOptions.test.ts

@@ -129,18 +129,9 @@ export const makeFetchOptions = (
     (typeof operation.context.fetchOptions === 'function'
       ? operation.context.fetchOptions()
       : operation.context.fetchOptions) || {};
-  if (extraOptions.headers) {
-    if (extraOptions.headers.forEach) {
-      (extraOptions.headers as Headers).forEach((value, key) => {
-        headers[key] = value;
-      });
-    } else {
-      for (const key in extraOptions.headers) {
-        headers[key.toLowerCase()] = extraOptions.headers[key];
-      }
-    }
-  }
-
+  if (extraOptions.headers)
+    for (const key in extraOptions.headers)
+      headers[key.toLowerCase()] = extraOptions.headers[key];
   const serializedBody = serializeBody(operation, body);
   if (typeof serializedBody === 'string' && !headers['content-type'])
     headers['content-type'] = 'application/json';