extra tests derived from Amazon docs

Amazon's docs provide some test values for signing the authorization header with dummy credentials. I have my own S3 library that uses these values so I went ahead and ported the tests over.

http://docs.amazonwebservices.com/AmazonS3/2006-03-01/dev/index.html?RESTAuthentication.html

One of the tests uses multiple values for the `X-Amz-Meta-ReviewedBy` header, which means the library must support the array form of headers (also supported by node) in order to preserve the ordering on values. I adjusted the `canonicalizeHeaders` function in auth.js to take this into account but I haven't added this support to other files/functions yet.

Amazon's date format in their test signing uses `+0000` instead of `GMT` like javascript does so I needed to allow for string dates in order to get the tests to pass. That doesn't sit quite right, to be honest, but it's just a teensy ternary op so it's probably OK?

If these changes look good I'll follow through with the header-as-array support for other files - let me know what you think.