refact: Revise unit tests for http config manager (#214)

Author: oakbani

lib/optimizely/config_manager/http_project_config_manager.rb

@@ -70,14 +70,16 @@ def initialize(
       @blocking_timeout = nil
       blocking_timeout(blocking_timeout)
       @last_modified = nil
-      @async_scheduler = AsyncScheduler.new(method(:fetch_datafile_config), @polling_interval, auto_update, @logger)
-      @async_scheduler.start! if start_by_default == true
-      @stopped = false
       @skip_json_validation = skip_json_validation
       @notification_center = notification_center.is_a?(Optimizely::NotificationCenter) ? notification_center : NotificationCenter.new(@logger, @error_handler)
       @config = datafile.nil? ? nil : DatafileProjectConfig.create(datafile, @logger, @error_handler, @skip_json_validation)
       @mutex = Mutex.new
       @resource = ConditionVariable.new
+      @async_scheduler = AsyncScheduler.new(method(:fetch_datafile_config), @polling_interval, auto_update, @logger)
+      # Start async scheduler in the end to avoid race condition where scheduler executes
+      # callback which makes use of variables not yet initialized by the main thread.
+      @async_scheduler.start! if start_by_default == true
+      @stopped = false
     end
 
     def ready?