Fix Params case handling in the index, sort and where func

This means that you can now do:

```
{{ range where .Site.Pages "Params.MYPARAM" "foo" }}
```

tpl/collections/collections_test.go

@@ -22,6 +22,8 @@ import (
 	"testing"
 	"time"
 
+	"github.com/gohugoio/hugo/common/maps"
+
 	qt "github.com/frankban/quicktest"
 	"github.com/gohugoio/hugo/common/loggers"
 	"github.com/gohugoio/hugo/config"
@@ -891,6 +893,15 @@ type TstX struct {
 	unexported string
 }
 
+type TstParams struct {
+	params maps.Params
+}
+
+func (x TstParams) Params() maps.Params {
+	return x.params
+
+}
+
 type TstXIHolder struct {
 	XI TstXI
 }