Assignment-like word shouldn't be subjected to tilde expansion in POSIX mode

[Oğuz]

See:

    $ set +k -o posix
    $ echo foo=~:~
    foo=~:/home/oguz

If I'm not misreading the standard `foo=~:~' should be printed
verbatim, all shells I have except bash does so.

This is reproducible on 5.1 alpha too.

Oğuz