common/collections: Fix type checking in Append

The fix introduced in Hugo `0.49.1` had an unintended side-effect in the `Append` func used in both `append` and `.Scratch.Add`.

This commit fixes that by loosen/fixing the type checking so concrete types can be appended to interface slices.

Fixes gohugoio#5303

common/collections/append.go

@@ -59,7 +59,8 @@ func Append(to interface{}, from ...interface{}) (interface{}, error) {
 
 	for _, f := range from {
 		fv := reflect.ValueOf(f)
-		if tot != fv.Type() {
+
+		if !fv.Type().AssignableTo(tot) {
 			return nil, fmt.Errorf("append element type mismatch: expected %v, got %v", tot, fv.Type())
 		}
 		tov = reflect.Append(tov, fv)

common/collections/append_test.go

@@ -43,7 +43,13 @@ func TestAppend(t *testing.T) {
 			tstSlicers{&tstSlicer{"a"},
 				&tstSlicer{"b"},
 				&tstSlicer{"c"}}},
+		{testSlicerInterfaces{&tstSlicerIn1{"a"}, &tstSlicerIn1{"b"}},
+			[]interface{}{&tstSlicerIn1{"c"}},
+			testSlicerInterfaces{&tstSlicerIn1{"a"}, &tstSlicerIn1{"b"}, &tstSlicerIn1{"c"}}},
 		// Errors
+		{testSlicerInterfaces{&tstSlicerIn1{"a"}, &tstSlicerIn1{"b"}},
+			[]interface{}{"c"},
+			false},
 		{"", []interface{}{[]string{"a", "b"}}, false},
 		// No string concatenation.
 		{"ab",

common/collections/slice_test.go

@@ -114,6 +114,7 @@ func TestSlice(t *testing.T) {
 		{[]interface{}{&tstSlicerIn1{"a"}, &tstSlicerIn2{"b"}}, testSlicerInterfaces{&tstSlicerIn1{"a"}, &tstSlicerIn2{"b"}}},
 		{[]interface{}{&tstSlicerIn1{"a"}, &tstSlicer{"b"}}, []interface{}{&tstSlicerIn1{"a"}, &tstSlicer{"b"}}},
 	} {
+
 		errMsg := fmt.Sprintf("[%d] %v", i, test.args)
 
 		result := Slice(test.args...)