add a true and false test and fix the implementation

src/bosh_aws_cpi/lib/cloud/aws/instance_manager.rb

@@ -34,7 +34,7 @@ def create(agent_id, stemcell_id, resource_pool, networks_spec, disk_locality, e
         instance.wait_for_running
         instance.attach_to_load_balancers(resource_pool['elbs'] || [])
         instance.update_routing_tables(resource_pool['advertised_routes'] || [])
-        instance.source_dest_check = resource_pool['source_dest_check'] || true
+        instance.source_dest_check = resource_pool.fetch('source_dest_check', true)
       rescue => e
         @logger.warn("Failed to configure instance '#{instance.id}': #{e.inspect}")
         begin

src/bosh_aws_cpi/spec/unit/instance_manager_spec.rb

@@ -91,7 +91,7 @@ module Bosh::AwsCloud
       it 'should ask AWS to create an instance in the given region, with parameters built up from the given arguments' do
         instance_manager = InstanceManager.new(ec2, registry, elb, param_mapper, block_device_manager, logger)
         allow(instance_manager).to receive(:get_created_instance_id).with("run-instances-response").and_return('i-12345678')
-        allow(aws_instance).to receive(:source_dest_check=).with(true)
+        allow(aws_instance).to receive(:source_dest_check=).with(false)
 
         expect(aws_client).to receive(:run_instances).with({ fake: 'instance-params', min_count: 1, max_count: 1 }).and_return("run-instances-response")
         instance_manager.create(
@@ -238,7 +238,7 @@ module Bosh::AwsCloud
         expect(aws_client).to receive(:run_instances).with({ fake: 'instance-params', min_count: 1, max_count: 1 }).and_return("run-instances-response")
 
         allow(ResourceWait).to receive(:for_instance).with(instance: aws_instance, state: :running)
-        allow(aws_instance).to receive(:source_dest_check=).with(true)
+        allow(aws_instance).to receive(:source_dest_check=).with(false)
         expect(logger).to receive(:warn).with(/IP address was in use/).once
 
         instance_manager.create(

src/bosh_aws_cpi/spec/unit/instance_spec.rb

@@ -169,10 +169,15 @@ module Bosh::AwsCloud
     end
 
     describe '#source_dest_check=' do
-      it 'propagates source_dest_check=' do
+      it 'propagates source_dest_check= true' do
         expect(aws_instance).to receive(:source_dest_check=).with(false)
         instance.source_dest_check = false
       end
+
+      it 'propagates source_dest_check= false' do
+        expect(aws_instance).to receive(:source_dest_check=).with(true)
+        instance.source_dest_check = true
+      end
     end
 
     describe '#attach_to_load_balancers' do