Improve s3

I fixed a few bugs in the s3 impelmentation, reworked the test a little bit and also had to do some minor changes in the spray-aws-client concerning encoding.

There is a new value in the props (describing whether the URL is encoded once or twice) which seems to [differ among amazon services](https://github.com/aws/aws-sdk-java/blob/master/src/main/java/com/amazonaws/auth/AWS4Signer.java#L73). Is this the right place to add such a flag?