fix: only add custom properties that aren't in the request object

Closes openapi-ts#1666

Author: jaredLunde

packages/openapi-fetch/src/index.js

@@ -14,7 +14,7 @@ class CustomRequest extends Request {
 
     // add custom parameters
     for (const key in init) {
-      if (!this[key]) {
+      if (!(key in this)) {
         this[key] = init[key];
       }
     }