Incorrect request signature

[zen4ever]

There is a bug in sorting query string parameters in case when parameter keys include dashes

https://github.com/sam-washington/requests-aws4auth/blob/master/requests_aws4auth/aws4auth.py#L661

Here parameters are sorted after they urlencoded, which puts them in a wrong order.

For example:

sorted("id=1000000161418039&id-type=receipt".split("&"))
>>> ['id-type=receipt', 'id=1000000161418039']

If you sort them by name, as AWS describes here http://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html, the order would be the following

sorted([tuple(pair.split("=")) for pair in "id=1000000161418039&id-type=receipt".split("&")], key=lambda x: x[0])
>>> [('id', '1000000161418039'), ('id-type', 'receipt')]

[tedder]

merged.