Previously newTestRequest() creates request object and returns
signature v4 signed request. In TestCopyObject(), its required to add
headers later to the request and sign the request.
This patch introduces two new functions
* signRequest(): signs request using given access/secret keys.
* newTestSignedRequest(): returns new request object signed with given
access/secret keys.
Fixes#2097
Change brings in a new signVerifyReader which provides a io.Reader
compatible reader, additionally implements Verify() function.
Verify() function validates the signature present in the incoming
request. This approach is choosen to avoid complexities involved
in using io.Pipe().
Thanks to Krishna for his inputs on this.
Fixes#2058Fixes#2054Fixes#2087
Signature calculation has now moved out from being a package to
top-level as a layered mechanism.
In case of payload calculation with body, go-routines are initiated
to simultaneously write and calculate shasum. Errors are sent
over the writer so that the lower layer removes the temporary files
properly.
