FIX crash when nofilter & compat is false

[ericzon]

First of all, than you for this lib, it's very useful for me. I've found a crash when somebody sets as option compat: false and doesn't specify defaultFilterRole or specify 'nofilter'.

As we can see on filter.js file, _getFilterKeys method:
var filter = {nofilter: null};

nofilter value is null, for that reason it crashes later on getReadFilterKeys method when:

filters.join(' ');