hugolib: Enhance .Param to permit arbitrarily nested parameter references

The Param method currently assumes that its argument is a single,
distinct, top-level key to look up in the Params map. This enhances the
Param method; it will now also attempt to see if the key can be
interpreted as a nested chain of keys to look up in Params.

Fixes #2598

hugolib/pageSort_test.go

@@ -20,7 +20,6 @@ import (
 	"testing"
 	"time"
 
-	"github.com/spf13/cast"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -121,11 +120,11 @@ func TestPageSortReverse(t *testing.T) {
 
 func TestPageSortByParam(t *testing.T) {
 	t.Parallel()
-	var k interface{} = "arbitrary"
+	var k interface{} = "arbitrarily.nested"
 	s := newTestSite(t)
 
 	unsorted := createSortTestPages(s, 10)
-	delete(unsorted[9].Params, cast.ToString(k))
+	delete(unsorted[9].Params, "arbitrarily")
 
 	firstSetValue, _ := unsorted[0].Param(k)
 	secondSetValue, _ := unsorted[1].Param(k)
@@ -137,7 +136,7 @@ func TestPageSortByParam(t *testing.T) {
 	assert.Equal(t, "xyz92", lastSetValue)
 	assert.Equal(t, nil, unsetValue)
 
-	sorted := unsorted.ByParam("arbitrary")
+	sorted := unsorted.ByParam("arbitrarily.nested")
 	firstSetSortedValue, _ := sorted[0].Param(k)
 	secondSetSortedValue, _ := sorted[1].Param(k)
 	lastSetSortedValue, _ := sorted[8].Param(k)
@@ -182,7 +181,9 @@ func createSortTestPages(s *Site, num int) Pages {
 	for i := 0; i < num; i++ {
 		p := s.newPage(filepath.FromSlash(fmt.Sprintf("/x/y/p%d.md", i)))
 		p.Params = map[string]interface{}{
-			"arbitrary": "xyz" + fmt.Sprintf("%v", 100-i),
+			"arbitrarily": map[string]interface{}{
+				"nested": ("xyz" + fmt.Sprintf("%v", 100-i)),
+			},
 		}
 
 		w := 5