URL encode string to sign.

As recommended at
http://s3.amazonaws.com/doc/s3-developer-guide/RESTAuthentication.html:

"You can also send a signature as a URL-encoded query-string parameter
in the URL for the request."

This manifests itself, for e.g., if the base64 encoded signature contains spaces
or other characters that need to be URL encoded. References JCLOUDS-200.