Route53 empty resource records

I'm not super familiar with what's going on here, so I'm not sure this is the right place to solve the issue. Passing an empty list as in `:resource_records => []` fails, since the `:resource_records` parameter should probably have been omitted. This change causes the `ChangeRequest` objects to be created without it, though it seems plausible to me that `ResourceRecordSet#resource_records` should return `nil` rather than `[]`.

This pulls in the two commits from #173 due to indentation changes.

Example logs showing the issue:

```
>> batch << rrset.new_delete_request
I, [2013-02-26T19:36:30.353518 #28203]  INFO -- aws-api: [AWS Route53 200 0.40468 0 retries] list_resource_record_sets(:hosted_zone_id=>"ABCDEFGHIJKLM",:start_record_name=>"example.com.",:start_record_type=>"A")
=> [#<AWS::Route53::DeleteRequest:0x7f1090e01418 @change_options={:type=>"A", :resource_records=>[], :alias_target=>{:dns_name=>"vips.example.com.", :evaluate_target_health=>true, :hosted_zone_id=>"ABCDEFGHIJKLM"}, :name=>"example.com."}, type"A", action"DELETE", name"example.com."]
>> batch.call
I, [2013-02-26T19:37:06.469893 #28203]  INFO -- aws-api: [AWS Route53 400 0.202559 0 retries] change_resource_record_sets(:change_batch=>{:changes=>[{:action=>"DELETE",:resource_record_set=>{:alias_target=>{:dns_name=>"vips.example.com.",:evaluate_target_health=>true,:hosted_zone_id=>"ABCDEFGHIJKLM"},:name=>"example.com.",:resource_records=>[],:type=>"A"}}]},:hosted_zone_id=>"ABCDEFGHIJKLM") AWS::Route53::Errors::InvalidInput Invalid XML ; cvc-complex-type.2.4.a: Invalid content was found starting with element 'ResourceRecords'. One of '{"https://route53.amazonaws.com/doc/2012-12-12/":SetIdentifier, "https://route53.amazonaws.com/doc/2012-12-12/":AliasTarget, "https://route53.amazonaws.com/doc/2012-12-12/":TTL}' is expected.
AWS::Route53::Errors::InvalidInput: Invalid XML ; cvc-complex-type.2.4.a: Invalid content was found starting with element 'ResourceRecords'. One of '{"https://route53.amazonaws.com/doc/2012-12-12/":SetIdentifier, "https://route53.amazonaws.com/doc/2012-12-12/":AliasTarget, "https://route53.amazonaws.com/doc/2012-12-12/":TTL}' is expected.
```