filab_strategy.rb

require 'omniauth-oauth2'

# this OmniAuth-Strategy uses the Keyrock Identity Management
# see http://catalogue.fiware.org/enablers/identity-management-keyrock
# The server url is from the public FIWARE Lab instance.
module OmniAuth
  module Strategies
    class FilabStrategy < OmniAuth::Strategies::OAuth2
      option :name, "filab"
      option :client_options, {
          :site => 'https://account.lab.fiware.org',
          :authorize_url => 'https://account.lab.fiware.org/oauth2/authorize/',
          :token_url => 'https://account.lab.fiware.org/oauth2/token'
      }

      uid { raw_info['id'].to_s }

      info do
        {
            'nickname' => raw_info['nickName'],
            'email' => raw_info['email'],
            'name' => raw_info['displayName'],
        }
      end

      extra do
        {:raw_info => raw_info}
      end

      def raw_info
        access_token.options[:mode] = :query
        @raw_info ||= access_token.get('user.json', params: { access_token: access_token.token }).parsed
      end
      
      def build_access_token
        Rails.logger.debug "Omniauth build access token"
        options.token_params.merge!(:headers => {'Authorization' => basic_auth_header })
        super
      end

      def basic_auth_header
        "Basic " + Base64.strict_encode64("#{options[:client_id]}:#{options[:client_secret]}")
      end      
    end
  end
end

@handylearn You could avoid the line 37 with a option like this

option :token_params, {
    :headers => {'Authorization' => "Basic " + Base64.strict_encode64("client_id:client_secret") }
}