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/sort.go

@@ -19,6 +19,7 @@ import (
 	"sort"
 	"strings"
 
+	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/tpl/compare"
 	"github.com/spf13/cast"
 )
@@ -75,11 +76,19 @@ func (ns *Namespace) Sort(seq interface{}, args ...interface{}) (interface{}, er
 			} else {
 				v := p.Pairs[i].Value
 				var err error
-				for _, elemName := range path {
+				for i, elemName := range path {
 					v, err = evaluateSubElem(v, elemName)
 					if err != nil {
 						return nil, err
 					}
+					if !v.IsValid() {
+						continue
+					}
+					// Special handling of lower cased maps.
+					if params, ok := v.Interface().(maps.Params); ok {
+						v = reflect.ValueOf(params.Get(path[i+1:]...))
+						break
+					}
 				}
 				p.Pairs[i].Key = v
 			}
@@ -89,6 +98,7 @@ func (ns *Namespace) Sort(seq interface{}, args ...interface{}) (interface{}, er
 		keys := seqv.MapKeys()
 		for i := 0; i < seqv.Len(); i++ {
 			p.Pairs[i].Value = seqv.MapIndex(keys[i])
+
 			if sortByField == "" {
 				p.Pairs[i].Key = keys[i]
 			} else if sortByField == "value" {
@@ -96,11 +106,19 @@ func (ns *Namespace) Sort(seq interface{}, args ...interface{}) (interface{}, er
 			} else {
 				v := p.Pairs[i].Value
 				var err error
-				for _, elemName := range path {
+				for i, elemName := range path {
 					v, err = evaluateSubElem(v, elemName)
 					if err != nil {
 						return nil, err
 					}
+					if !v.IsValid() {
+						continue
+					}
+					// Special handling of lower cased maps.
+					if params, ok := v.Interface().(maps.Params); ok {
+						v = reflect.ValueOf(params.Get(path[i+1:]...))
+						break
+					}
 				}
 				p.Pairs[i].Key = v
 			}
@@ -135,6 +153,7 @@ func (p pairList) Less(i, j int) bool {
 			// can only call Interface() on valid reflect Values
 			return sortComp.Lt(iv.Interface(), jv.Interface())
 		}
+
 		// if j is invalid, test i against i's zero value
 		return sortComp.Lt(iv.Interface(), reflect.Zero(iv.Type()))
 	}