Explicitly use path?query for V4 signatures

The setting of `url.Opaque` in https://github.com/crowdmob/goamz/commit/be331c13a28a2c3ba0c216c52a5401388d72d235#diff-7db3cb93944d57b2ffc803281c906018R960 causes `url.RequestURI()` to return a full url e.g. `scheme://host/path?query` rather than `path?query` which is invalid for the V4 signature.  `path?query` must be used.