Fix map filter with slice of structs

The previous map implementation worked only for slices of maps.
By using the library's `values` package this implementation should work
with all compatible types.

filters/standard_filters.go

@@ -47,9 +47,11 @@ func AddStandardFilters(fd FilterDictionary) { // nolint: gocyclo
 		return append(append(result, a...), b...)
 	})
 	fd.AddFilter("join", joinFilter)
-	fd.AddFilter("map", func(a []map[string]interface{}, key string) (result []interface{}) {
+	fd.AddFilter("map", func(a []interface{}, key string) (result []interface{}) {
+		keyValue := values.ValueOf(key)
 		for _, obj := range a {
-			result = append(result, obj[key])
+			value := values.ValueOf(obj)
+			result = append(result, value.PropertyValue(keyValue).Interface())
 		}
 		return result
 	})

filters/standard_filters_test.go

@@ -60,6 +60,8 @@ var filterTests = []struct {
 	{`map_slice_dup | join`, `a a b`},
 	{`map_slice_dup | uniq | join`, `a b`},
 
+	{`struct_slice | map: "str" | join`, `a b c`},
+
 	// date filters
 	{`article.published_at | date`, "Fri, Jul 17, 15"},
 	{`article.published_at | date: "%a, %b %d, %y"`, "Fri, Jul 17, 15"},
@@ -237,6 +239,13 @@ var filterTestBindings = map[string]interface{}{
 		{"name": "page 6"},
 		{"name": "page 7", "category": "technology"},
 	},
+	"struct_slice": []struct {
+		Str string `liquid:"str"`
+	}{
+		{Str: "a"},
+		{Str: "b"},
+		{Str: "c"},
+	},
 }
 
 func TestFilters(t *testing.T) {