Added subscriber check method for connection class and added Gemfile

Gemfile

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+source 'https://rubygems.org'
+
+# Specify your gem's dependencies in stomp.gemspec
+gemspec

lib/stomp/client.rb

@@ -289,7 +289,7 @@ def closed?()
     #     Stomp::Client.new().subscriber?('/queue/queueName')                                     => true
     #     Stomp::Client.new().subscriber?('/queue/Consumer.subscriber1.VirtualTopic.topicName')   => true
     #
-    def subscriber?(destination, headers = {})
+    def subscribed?(destination, headers = {})
       headers = headers.symbolize_keys
       headers = headers.merge(:id => build_subscription_id(destination, headers))
       !@listeners[headers[:id]].nil?

lib/stomp/connection.rb

@@ -371,6 +371,13 @@ def subscribe(destination, headers = {}, subId = nil)
       transmit(Stomp::CMD_SUBSCRIBE, headers)
     end
 
+    def subscribed?(destination, subId = nil)
+      return false unless @reliable
+
+      subId = destination if subId.nil?
+      @subscriptions[subId].present?
+    end
+
     # Unsubscribe from a destination.   A subscription name is required.
     # For Stomp 1.1+ a session unique subscription ID is also required.
     def unsubscribe(destination, headers = {}, subId = nil)

spec/client_spec.rb

@@ -471,8 +471,8 @@ def original_headers
       it_behaves_like 'argument-safe method'
     end
 
-    describe '#subscriber?' do
-      subject { @client.subscriber?('/topic/topicName') }
+    describe '#subscribed?' do
+      subject { @client.subscribed?('/topic/topicName') }
 
       context 'When subscription is present' do
         before {