AWS::S3::Client#put_bucket_website now omits empty routing rules.

This resolves an issue where passing `:routing_rules => []` to the
client would result in an error returned from Amazon S3.  S3 does
not accept `<RoutingRules/>`.

Fixes #308
Merge pull request #310 from aws/s3-website-fix

AWS::S3::Client#put_bucket_website now omits empty routing rules.