Missing required parameter: assertion

Hey guys, Here's my code:

  client = OAuth2::Client.new('', '',
    site: 'https://accounts.google.com',
    authorize_url: '/o/oauth2/auth',
    token_url: '/o/oauth2/token',
    auth_scheme: :request_body
  )

  encoding = {
    algorithm: 'RS256',
    key: self.get_certificate_from_assembly.key,
  }

  now = DateTime.now
  claimset = {
    iss: Google::Settings.service_account_email,
    scope: "https://www.googleapis.com/auth/youtubepartner",
    aud: "https://accounts.google.com/o/oauth2/token",
    iat: now.to_i,
    exp: (now + 10.minutes).to_i,
  }

  access = client.assertion.get_token(claimset, encoding)
  token = access.token

Whenever I use the gem directly from rubygems, I get this error:

OAuth2::Error: invalid_request: Missing required parameter: assertion
{
  "error": "invalid_request",
  "error_description": "Missing required parameter: assertion"
}

However, when I use it from github gem 'oauth2', git: 'git@github.com:oauth-xx/oauth2.git' the same code is passing and I'm getting the token.

Is the rubygems version outdated ? should I keep using the git version until 2.0.0 is released ?