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" }}
```

common/maps/maps.go

@@ -25,26 +25,45 @@ import (
 // recursively.
 // Notes:
 // * This will modify the map given.
-// * Any nested map[interface{}]interface{} will be converted to map[string]interface{}.
-func ToLower(m map[string]interface{}) {
+// * Any nested map[interface{}]interface{} will be converted to Params.
+func ToLower(m Params) {
 	for k, v := range m {
+		var retyped bool
 		switch v.(type) {
 		case map[interface{}]interface{}:
-			v = cast.ToStringMap(v)
-			ToLower(v.(map[string]interface{}))
+			var p Params = cast.ToStringMap(v)
+			v = p
+			ToLower(p)
+			retyped = true
 		case map[string]interface{}:
-			ToLower(v.(map[string]interface{}))
+			var p Params = v.(map[string]interface{})
+			v = p
+			ToLower(p)
+			retyped = true
 		}
 
 		lKey := strings.ToLower(k)
-		if k != lKey {
+		if retyped || k != lKey {
 			delete(m, k)
 			m[lKey] = v
 		}
-
 	}
 }
 
+func ToStringMapE(in interface{}) (map[string]interface{}, error) {
+	switch in.(type) {
+	case Params:
+		return in.(Params), nil
+	default:
+		return cast.ToStringMapE(in)
+	}
+}
+
+func ToStringMap(in interface{}) map[string]interface{} {
+	m, _ := ToStringMapE(in)
+	return m
+}
+
 type keyRename struct {
 	pattern glob.Glob
 	newKey  string