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" }}
```

resources/page/page_nop.go

@@ -300,7 +300,7 @@ func (p *nopPage) Param(key interface{}) (interface{}, error) {
 	return nil, nil
 }
 
-func (p *nopPage) Params() map[string]interface{} {
+func (p *nopPage) Params() maps.Params {
 	return nil
 }