Avoid changing cv-qualifier on override, should fix msvc warning

Status Job ID Name Coverage
  Test
passed #67575
VS2015
VS2015-32-bit

00:01:08

passed #67574
VS2015
VS2015-64-bit

00:01:17

passed #67573
VS2017
VS2017-32-bit

00:01:12

passed #67572
VS2015
VS2017-64-bit

00:01:18

passed #67569
Linux
clang-c++11

00:03:37

passed #67571
Linux
cppcheck

00:00:41

passed #67576
Linux
doc

00:00:09

passed #67568
Linux
gcc-c++11

00:03:58

passed #67570
Apple
macos-c++11

00:03:01