handler/PUT: Handle signature verification through a custom reader. (#2066)

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 #2058
Fixes #2054
Fixes #2087

auth-handler.go

@@ -27,10 +27,6 @@ import (
 	"strings"
 )
 
-// http Header "x-amz-content-sha256" == "UNSIGNED-PAYLOAD" indicates that the
-// client did not calculate sha256 of the payload.
-const unsignedPayload = "UNSIGNED-PAYLOAD"
-
 // Verify if the request http Header "x-amz-content-sha256" == "UNSIGNED-PAYLOAD"
 func isRequestUnsignedPayload(r *http.Request) bool {
 	return r.Header.Get("x-amz-content-sha256") == unsignedPayload
@@ -136,7 +132,9 @@ func isReqAuthenticated(r *http.Request) (s3Error APIErrorCode) {
 	r.Body = ioutil.NopCloser(bytes.NewReader(payload))
 	validateRegion := true // Validate region.
 	var sha256sum string
-	if skipSHA256Calculation(r) {
+	// Skips calculating sha256 on the payload on server,
+	// if client requested for it.
+	if skipContentSha256Cksum(r) {
 		sha256sum = unsignedPayload
 	} else {
 		sha256sum = hex.EncodeToString(sum256(payload))