test and fix for Grid#delete

lib/mongo/gridfs/grid.rb

@@ -78,7 +78,7 @@ module Mongo
     # @return [Boolean]
     def delete(id)
       @files.remove({"_id" => id})
-      @chunks.remove({"_id" => id})
+      @chunks.remove({"files_id" => id})
     end
 
     private

test/grid_test.rb

@@ -42,6 +42,7 @@ class GridTest < Test::Unit::TestCase
         assert_raise GridError do
           @grid.get(@id)
         end
+        assert_equal nil, @db['test-fs']['chunks'].find_one({:files_id => @id})
       end
     end