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
 }
 

resources/page/site.go

@@ -17,6 +17,8 @@ import (
 	"html/template"
 	"time"
 
+	"github.com/gohugoio/hugo/common/maps"
+
 	"github.com/gohugoio/hugo/config"
 
 	"github.com/gohugoio/hugo/common/hugo"
@@ -39,7 +41,7 @@ type Site interface {
 	Taxonomies() interface{}
 	LastChange() time.Time
 	Menus() navigation.Menus
-	Params() map[string]interface{}
+	Params() maps.Params
 	Data() map[string]interface{}
 }
 
@@ -107,7 +109,7 @@ func (t testSite) BaseURL() template.URL {
 	return ""
 }
 
-func (t testSite) Params() map[string]interface{} {
+func (t testSite) Params() maps.Params {
 	return nil
 }
 

resources/page/testhelpers_test.go

@@ -370,7 +370,7 @@ func (p *testPage) Param(key interface{}) (interface{}, error) {
 	return resource.Param(p, nil, key)
 }
 
-func (p *testPage) Params() map[string]interface{} {
+func (p *testPage) Params() maps.Params {
 	return p.params
 }
 

resources/resource.go

@@ -30,9 +30,9 @@ import (
 	"github.com/pkg/errors"
 
 	"github.com/gohugoio/hugo/common/hugio"
+	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/resources/page"
 	"github.com/gohugoio/hugo/resources/resource"
-
 	"github.com/spf13/afero"
 
 	"github.com/gohugoio/hugo/helpers"
@@ -228,7 +228,7 @@ func (l *genericResource) Name() string {
 	return l.name
 }
 
-func (l *genericResource) Params() map[string]interface{} {
+func (l *genericResource) Params() maps.Params {
 	return l.params
 }
 

resources/resource/params.go

@@ -19,12 +19,16 @@ import (
 	"github.com/spf13/cast"
 )
 
-func Param(r ResourceParamsProvider, fallback map[string]interface{}, key interface{}) (interface{}, error) {
+func Param(r ResourceParamsProvider, fallback maps.Params, key interface{}) (interface{}, error) {
 	keyStr, err := cast.ToStringE(key)
 	if err != nil {
 		return nil, err
 	}
 
+	if fallback == nil {
+		return maps.GetNestedParam(keyStr, ".", r.Params())
+	}
+
 	return maps.GetNestedParam(keyStr, ".", r.Params(), fallback)
 
 }

resources/resource/resourcetypes.go

@@ -14,6 +14,7 @@
 package resource
 
 import (
+	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/media"
 	"github.com/gohugoio/hugo/resources/images/exif"
@@ -85,7 +86,7 @@ type ResourceMetaProvider interface {
 
 type ResourceParamsProvider interface {
 	// Params set in front matter for this resource.
-	Params() map[string]interface{}
+	Params() maps.Params
 }
 
 type ResourceDataProvider interface {

resources/resource_metadata.go

@@ -129,7 +129,7 @@ func AssignMetadata(metadata []map[string]interface{}, resources ...resource.Res
 
 				params, found := meta["params"]
 				if found {
-					m := cast.ToStringMap(params)
+					m := maps.ToStringMap(params)
 					// Needed for case insensitive fetching of params values
 					maps.ToLower(m)
 					ma.updateParams(m)

resources/transform.go

@@ -26,11 +26,11 @@ import (
 
 	bp "github.com/gohugoio/hugo/bufferpool"
 
-	"github.com/gohugoio/hugo/resources/internal"
-
 	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/gohugoio/hugo/common/hugio"
+	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/resources/internal"
 	"github.com/gohugoio/hugo/resources/resource"
 
 	"github.com/gohugoio/hugo/media"
@@ -200,7 +200,7 @@ func (r *resourceAdapter) Name() string {
 	return r.target.Name()
 }
 
-func (r *resourceAdapter) Params() map[string]interface{} {
+func (r *resourceAdapter) Params() maps.Params {
 	r.init(false, false)
 	return r.target.Params()
 }