Update lib/mongoid_search/util.rb

If one of the fields in "search_in" is a method, then the old-vay "klass[field]" returns nil.
With this fix it executes the method and uses its result. Also, for regular fields it also works fine.
Merge pull request #49 from borodiychuk/patch-1

If one of the fields in search_in is a method, then the old-way klass[field] returns nil.
With this fix it executes the method and uses its result. Also, for regular fields it also works fine.