401 Unauthorized from Twitter

[MrIngelborn]

When sending the request for the initial token to Twitters API according to the instructions given by Twitter Sign-In dev docs, with help of oauth, omniauth and omniauth-twitter gems, we get a 401 Unauthorized with the following header:

{
    "cache-control" : [
        "no-cache, no-store, must-revalidate, pre-check=0, post-check=0"
    ],"connection":[
        "close"
    ],
    "content-disposition":[
        "attachment; filename=json.json"
    ],
    "content-length":[
        "89"
    ],
    "content-type":[
        "application/json; charset=utf-8"
    ],
    "date":[
        "Wed, 09 Dec 2015 14:06:22 GMT"
    ],
    "expires":[
        "Tue, 31 Mar 1981 05:00:00 GMT"
    ],
    "last-modified":[
        "Wed, 09 Dec 2015 14:06:22 GMT"
    ],
    "pragma":[
        "no-cache"
    ],
    "server":[
        "tsa_b"
    ],
    "set-cookie":[
        "guest_id=v1%3A144966998241171180; Domain=.twitter.com; Path=/; Expires=Fri, 08-Dec-2017 14:06:22 UTC"
    ],
    "status":[
        "401 Unauthorized"
    ],
    "strict-transport-security":[
        "max-age=631138519"
    ],
    "www-authenticate":[
        "OAuth realm=\"https://api.twitter.com\""
    ],
    "x-connection-hash":[
        "abe5ee55f325a37df9b6737ed850e36a"
    ],
    "x-content-type-options":[
        "nosniff"
    ],
    "x-frame-options":["SAMEORIGIN"],
    "x-response-time":["8"],
    "x-transaction":["7ecc34ff643e2eb4"],
    "x-twitter-response-tags":["BouncerCompliant"],
    "x-xss-protection":["1; mode=block"]
}