fix: manage Headers instances in the JS kit (#235)

kits/javascript/shims/types/headers.js

@@ -9,7 +9,12 @@ class Headers {
 
     // Initialize the headers
     for (const key in initialHeaders) {
-      headers[key] = initialHeaders[key];
+      let value = initialHeaders[key];
+
+      // Allow only string values
+      if (typeof value === "string") {
+        headers[key] = value;
+      }
     }
 
     this.headers = headers;

kits/javascript/shims/types/response.js

@@ -10,7 +10,15 @@ import httpStatus from "http-status";
 class Response {
   constructor(body, options = {}) {
     this.body = body;
-    this.headers = new Headers(options.headers || {});
+
+    if (options.headers instanceof Headers) {
+      this.headers = options.headers;
+    } else if (options.headers instanceof Object) {
+      this.headers = new Headers(options.headers);
+    } else {
+      this.headers = new Headers({});
+    }
+
     this.status = options.status || 200;
     this.statusText = options.statusText || httpStatus[this.status];
   }