fix EC2 Signature Version 4 calculation, in the case of POST

When calculating the AWS Signature Version 4, in the case of POST,
We need to set the CanonicalQueryString to an empty string. this
follows the implementation of the AWS and boto clients.

Change-Id: Iad4e392119067e246c7b77009da3fef48d251382
Closes-Bug: 1360892
Merge "fix EC2 Signature Version 4 calculation, in the case of POST"