fix: webui pass correct arguments to request() in putBucketPolicy

The fifth parameter of request() is useAdminEndpoint (boolean), but
putBucketPolicy was passing a Content-Type header object instead.
This caused useAdminEndpoint to be truthy and contentType to default
to 'application/xml' instead of 'application/json'.

Fixed by passing false for useAdminEndpoint and 'application/json'
as the contentType argument.

Fixes #1928

webui/web/js/api.js

@@ -1846,9 +1846,7 @@ ${tagsXml}
    */
   async putBucketPolicy(bucket, policy) {
     const policyJson = JSON.stringify(policy);
-    await this.request('PUT', `/${bucket}`, { policy: '' }, policyJson, {
-      'Content-Type': 'application/json'
-    });
+    await this.request('PUT', `/${bucket}`, { policy: '' }, policyJson, false, 'application/json');
   }
 
   /**