fix: only add custom properties that aren't in the request object (openapi-ts#1672)

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

Closes openapi-ts#1666

* add changeset

Author: jaredLunde

.changeset/wet-apples-hide.md

@@ -0,0 +1,5 @@
+---
+"openapi-fetch": patch
+---
+
+Fixes issue where native properties were not excluded from custom properties in the CustomRequest class

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];
       }
     }