Simplify the code a bit inside ensure_threads_running!

Author: cstyles

lib/kafka/async_producer.rb

@@ -155,11 +155,8 @@ def ensure_threads_running!
       return if worker_thread_alive? && timer_thread_alive?
 
       @thread_mutex.synchronize do
-        @worker_thread = nil unless worker_thread_alive?
-        @worker_thread ||= Thread.new { @worker.run }
-
-        @timer_thread = nil unless timer_thread_alive?
-        @timer_thread ||= Thread.new { @timer.run }
+        @worker_thread = Thread.new { @worker.run } unless worker_thread_alive?
+        @timer_thread = Thread.new { @timer.run } unless timer_thread_alive?
       end
     end