setOption(Memcached::OPT_PREFIX_KEY => false) should not return true

[SjonHortensius]

Previously OPT_PREFIX_KEY could be set to false, which would be equivalent to specifying an empty string. Since recently this no longer works; setOption returns true but the prefix remains unchanged.

[SjonHortensius]

Possibly related; setOption(Memcached::OPT_PREFIX_KEY, '') doesn't seem to work either; the prefix remains the same. This is all on the php7 branch; using libmemcached 1.0.18

[SjonHortensius]

Fixed in #294