Fix uniq filter with string input

test/integration/standard_filter_test.rb

@@ -170,6 +170,7 @@ class StandardFiltersTest < Minitest::Test
   end
 
   def test_uniq
+    assert_equal ["foo"], @filters.uniq("foo")
     assert_equal [1, 3, 2, 4], @filters.uniq([1, 1, 3, 2, 3, 1, 4, 3, 2, 1])
     assert_equal [{ "a" => 1 }, { "a" => 3 }, { "a" => 2 }], @filters.uniq([{ "a" => 1 }, { "a" => 3 }, { "a" => 1 }, { "a" => 2 }], "a")
     testdrop = TestDrop.new