Ensure 0 is always the default for the individual exception counters in ActiveJob

Some adapters like Resque that use Redis, convert the Ruby hash with a
default value, Hash.new(0), into a regular hash without a default value
after serializing, storing and deserializing. This raises an error when
we try to access a missing exception key. A simple solution is not to
rely on the hash's default value, and provide a default as alternative
when accessing it instead.

activejob/lib/active_job/core.rb

@@ -81,7 +81,7 @@ module ActiveJob
       @queue_name = self.class.queue_name
       @priority   = self.class.priority
       @executions = 0
-      @exception_executions = Hash.new(0)
+      @exception_executions = {}
     end
 
     # Returns a hash with the job data that can safely be passed to the

activejob/lib/active_job/exceptions.rb

@@ -50,8 +50,8 @@ module ActiveJob
       def retry_on(*exceptions, wait: 3.seconds, attempts: 5, queue: nil, priority: nil)
         rescue_from(*exceptions) do |error|
           # Guard against jobs that were persisted before we started having individual executions counters per retry_on
-          self.exception_executions ||= Hash.new(0)
-          self.exception_executions[exceptions.to_s] += 1
+          self.exception_executions ||= {}
+          self.exception_executions[exceptions.to_s] = (exception_executions[exceptions.to_s] || 0) + 1
 
           if exception_executions[exceptions.to_s] < attempts
             retry_job wait: determine_delay(wait), queue: queue, priority: priority, error: error