metrics: Adjust the howSimilar logic vs strings

Also add a test.

metrics/metrics_test.go

@@ -17,6 +17,8 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/gohugoio/hugo/resources/page"
+
 	"github.com/stretchr/testify/require"
 )
 
@@ -40,6 +42,12 @@ func TestSimilarPercentage(t *testing.T) {
 
 }
 
+func TestSimilarPercentageNonString(t *testing.T) {
+	assert := require.New(t)
+	assert.Equal(100, howSimilar(page.NopPage, page.NopPage))
+	assert.Equal(90, howSimilar(page.Pages{}, page.Pages{}))
+}
+
 func BenchmarkHowSimilar(b *testing.B) {
 	s1 := "Hugo is cool and " + strings.Repeat("fun ", 10) + "!"
 	s2 := "Hugo is cool and " + strings.Repeat("cool ", 10) + "!"