Expires time is calculated twice when using expires_in and can cause signature authentication problems

[loe]

@bmo figured this out, I'm just the messenger.

The build method in QueryString calls #expires, and so does #encoded_canonical.

 # Keep in alphabetical order
 def build
   "AWSAccessKeyId=#{access_key_id}&Expires=#{expires}&Signature=#{encoded_canonical}"
 end

If these calls lie on separate sides of a second tick the signature will be wrong. The solution is to memoize the first call so encoded_canonical uses the same value.