Filter Expires header from user metadata (#7269)

Instead save it as a struct field in ObjectInfo as it is
a standard HTTP header - Fixes minio/mc#2690

cmd/fs-v1-metadata_test.go

@@ -38,6 +38,9 @@ func TestFSV1MetadataObjInfo(t *testing.T) {
 	if objInfo.IsDir {
 		t.Fatal("Unexpected object info value for IsDir", objInfo.IsDir)
 	}
+	if !objInfo.Expires.IsZero() {
+		t.Fatal("Unexpected object info value for Expires ", objInfo.Expires)
+	}
 }
 
 // TestReadFSMetadata - readFSMetadata testing with a healthy and faulty disk