add group robot signature calculate method

lib/dingtalk_sdk.rb

@@ -73,7 +73,7 @@ module DingtalkSdk
                   end
 
       signature_str = OpenSSL::HMAC.digest('SHA256', app_secret, timestamp.to_s)
-      signature_str_base64 = Base64.encode64(signature_str).strip
+      signature_str_base64 = Base64.strict_encode64(signature_str)
 
       Signature.new(signature_str_base64)
     end

lib/dingtalk_sdk/robot.rb

+# frozen_string_literal: true
+
+require 'base64'
+require 'openssl'
+require 'dingtalk_sdk'
+require 'dingtalk_sdk/core'
+require 'dingtalk_sdk/request_url'
+require 'active_support/core_ext/time/zones'
+
+module DingtalkSdk
+  module Robot
+    extend DingtalkSdk::Core
+
+    def self.calculate_signature(secret, timestamp = nil)
+      timestamp = Time.zone.now.to_i if timestamp.nil?
+
+      origin_str = [timestamp * 1000, secret].join("\n")
+      signature_str = OpenSSL::HMAC.digest('SHA256', secret, origin_str)
+      signature_str_base64 = Base64.strict_encode64(signature_str)
+
+      Signature.new(signature_str_base64)
+    end
+  end
+end

spec/dingtalk_sdk/robot_spec.rb

+# frozen_string_literal: true
+
+require 'dingtalk_sdk'
+require 'dingtalk_sdk/robot'
+
+RSpec.describe DingtalkSdk::Robot do
+  it 'should calculate correct signature' do
+    timestamp = 1_603_868_410
+    secret = 'this is secret'
+    sign = DingtalkSdk::Robot.calculate_signature(secret, timestamp)
+
+    expect(sign.url_encoded).to eq('3E1RZgQrn1ZLgQGU4C4n4SMei%2BObeZVym5BbqrwOJPs%3D')
+  end
+end