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.

lib/mongoid_search/util.rb

@@ -29,7 +29,7 @@ def self.keywords(klass, field)
         end
       end
     else
-      value = klass[field]
+      value = klass.send(field)
       value = value.join(' ') if value.respond_to?(:join)
       normalize_keywords(value) if value
     end