Use "strict checking" with in_array() and 0.

See [This PHP snippet](http://viper-7.com/DWpx92) -- `in_array()` will always return `true` when zero is passed as the needle and the values in the array are all strings UNLESS you tell it to use strict checking (pass `true` for the third argument). 
Without this fix, `pmpro_hasMembershipLevel` will return `true` for anonymous users when you pass in a membership level name instead of an ID.