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.
Better fix for the delete_if bug

Actually, this works better since #set checks for [[]].
Merge pull request #190 from mferrier/patch-1

CORSRuleCollection delete_if can delete all rules