using set +o posix unsets POSIXLY_CORRECT, even when POSIXLY_CORRECT is a read only variable

[nerdopolis]

I tested on
4.2.45(1)-release

basically I have tested 
nerdopolis@nerdopolis:~$ readonly POSIXLY_CORRECT=1
nerdopolis@nerdopolis:~$ set +o posix
nerdopolis@nerdopolis:~$ echo $POSIXLY_CORRECT

nerdopolis@nerdopolis:~$

Although I will admit I'm not sure if it's supposed to do that or not