CORSRuleCollection delete_if can delete all rules

If CORSRuleCollection#delete_if matched all the rules, you'd get:

```
  ArgumentError - expected one or more rules:
    (gem) aws-sdk-1.8.3.1/lib/aws/s3/cors_rule_collection.rb:101:in `set'
```
So if all the rules are deleted, call #clear instead.