Make error handler change fully backwards compatible

The change that Fixes mperham/sidekiq#3673 actually broke any existing
exception handlers due to differences in expected parameter count. This
fixes it explicitly which normally seems like it won't matter but there
are random monitoring gems that install their own sidekiq exception
handlers that I don't want to break.

lib/sidekiq/exception_handler.rb

@@ -24,7 +24,13 @@ module Sidekiq
     def handle_exception(ex, ctxHash={}, options={})
       Sidekiq.error_handlers.each do |handler|
         begin
-          handler.call(ex, ctxHash, options)
+          arity = handler.method(:call).arity
+          # new-style three argument method or fully variable arguments
+          if arity == -3 || arity == -1
+            handler.call(ex, ctxHash, options)
+          else
+            handler.call(ex, ctxHash)
+          end
         rescue => ex
           Sidekiq.logger.error "!!! ERROR HANDLER THREW AN ERROR !!!"
           Sidekiq.logger.error ex
@@ -32,6 +38,5 @@ module Sidekiq
         end
       end
     end
-
   end
 end

test/test_processor.rb

@@ -141,6 +141,34 @@ class TestProcessor < Sidekiq::Test
         assert_instance_of StandardError, errors.last[:exception]
         assert_equal :error, errors.last[:options][:level]
       end
+
+      class TestHandler
+        attr_reader :count
+
+        def initialize
+          @count = 0
+        end
+
+        def call(exc, ctx)
+          @count += 1
+        end
+      end
+
+      it 'can call two argument exception handlers' do
+        handler = TestHandler.new
+        Sidekiq.error_handlers << handler
+        job_hash = { 'class' => MockWorker.to_s, 'args' => ['boom'] }
+        msg = Sidekiq.dump_json(job_hash)
+        @processor.instance_variable_set(:'@reloader', proc { raise TEST_EXCEPTION })
+        job = work(msg)
+        begin
+          @processor.instance_variable_set(:'@job', job)
+          @processor.process(job)
+        rescue TestException
+        end
+        assert_equal 1, handler.count
+        Sidekiq.error_handlers.pop
+      end
     end
 
     describe 'acknowledgement' do