tpl/collections: Fix apply with namespaced template funcs

We changed the signature to `func(...interface{}) (interface{}, error)` some time ago, but sadly we had no test for this for `apply`. Now we do.

Fixes #9393

hugolib/template_test.go

@@ -460,7 +460,6 @@ complex: 80: 80
 
 // Issue 7528
 func TestPartialWithZeroedArgs(t *testing.T) {
-
 	b := newTestSitesBuilder(t)
 	b.WithTemplatesAdded("index.html",
 		` 
@@ -483,7 +482,6 @@ X123X
 X123X
 X123X
 `)
-
 }
 
 func TestPartialCached(t *testing.T) {
@@ -757,3 +755,20 @@ This is single main
 `,
 	)
 }
+
+// Issue 9393.
+func TestApplyWithNamespace(t *testing.T) {
+	b := newTestSitesBuilder(t)
+
+	b.WithTemplates(
+		"index.html", `
+{{ $b := slice " a " "     b "   "       c" }}		
+{{ $a := apply $b "strings.Trim" "." " " }}
+a: {{ $a }}
+`,
+	).WithContent("p1.md", "")
+
+	b.Build(BuildCfg{})
+
+	b.AssertFileContent("public/index.html", `a: [a b c]`)
+}