Ambiguous OAuth request error

[phamdacloc]

I'm having a really difficult time trying to get the OAuth 1.0a plugin to authenticate properly with iOS client. On iOS, i'm using OAuthSwift library which are currently being used by thousands of developers and client apps to authenticate the users in LinkedIn, Facebook, Dropbox, bitbucket, Instagram, Github...etc. I was able to authenticate myself with LinkedIn oauth 1.0a server. However, i kept on receiving the ambiguous RequestError when trying to send request to http://example.com/oauth1/request endpoint on the Wordpress site of mine.

My Wordpress http (not https) site was a brand new install with version 4.6.1. I then installed WordPress REST API (Version 2) plugin with version 2.0-beta15. Lastly, I installed WordPress REST API - OAuth 1.0a Server plugin with version 0.3.0.

Sending the request to http://faifo.us/oauth1/request endpoint using Postman works fine if I tick the checkbox Add empty params to signature.

I can't really understand why OAuth 1.0a server on the Wordpress site is not working. Any tips on what else I can try to debug this problem further would be greatly appreciated.