Fix s3cmd issue with ACL handling (#5999)

With the implementation of dummy GET ACL handlers,
tools like s3cmd perform few operations which causes
the ACL call to be invoked. Make sure that in our
router configuration GET?acl comes before actual
GET call to facilitate this dummy call.

cmd/acl-handlers.go

@@ -82,7 +82,9 @@ func (api objectAPIHandlers) GetBucketACLHandler(w http.ResponseWriter, r *http.
 	acl := &accessControlPolicy{}
 	acl.AccessControlList.Grants = append(acl.AccessControlList.Grants, grant{
 		Grantee: grantee{
-			Type: "CanonicalUser",
+			XMLNS:  "http://www.w3.org/2001/XMLSchema-instance",
+			XMLXSI: "CanonicalUser",
+			Type:   "CanonicalUser",
 		},
 		Permission: "FULL_CONTROL",
 	})
@@ -128,7 +130,9 @@ func (api objectAPIHandlers) GetObjectACLHandler(w http.ResponseWriter, r *http.
 	acl := &accessControlPolicy{}
 	acl.AccessControlList.Grants = append(acl.AccessControlList.Grants, grant{
 		Grantee: grantee{
-			Type: "CanonicalUser",
+			XMLNS:  "http://www.w3.org/2001/XMLSchema-instance",
+			XMLXSI: "CanonicalUser",
+			Type:   "CanonicalUser",
 		},
 		Permission: "FULL_CONTROL",
 	})

cmd/api-router.go

@@ -69,6 +69,8 @@ func registerAPIRouter(router *mux.Router) {
 		bucket.Methods("POST").Path("/{object:.+}").HandlerFunc(httpTraceAll(api.NewMultipartUploadHandler)).Queries("uploads", "")
 		// AbortMultipartUpload
 		bucket.Methods("DELETE").Path("/{object:.+}").HandlerFunc(httpTraceAll(api.AbortMultipartUploadHandler)).Queries("uploadId", "{uploadId:.*}")
+		// GetObjectACL - this is a dummy call.
+		bucket.Methods("GET").Path("/{object:.+}").HandlerFunc(httpTraceHdrs(api.GetObjectACLHandler)).Queries("acl", "")
 		// GetObject
 		bucket.Methods("GET").Path("/{object:.+}").HandlerFunc(httpTraceHdrs(api.GetObjectHandler))
 		// CopyObject