Fix for page.GetParam() for JSON and TOML maps

Setting per-page Blackfriday angledQuotes did not work
with TOML or JSON front matter, but it does work with YAML.

It turns out that page.Params("blackfriday") returns
type map[interface{}]interface{} for YAML, but
type map[string]interface{} for JSON and TOML.

This patch updates page.GetParam() to catch the latter,
with an error message if page.GetParam() does not recognize
a type.  A test is also added.

hugolib/page.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"github.com/spf13/hugo/helpers"
 	"github.com/spf13/hugo/parser"
+	"reflect"
 
 	"github.com/spf13/cast"
 	"github.com/spf13/hugo/hugofs"
@@ -522,9 +523,13 @@ func (page *Page) GetParam(key string) interface{} {
 		return cast.ToTime(v)
 	case []string:
 		return helpers.SliceToLower(v.([]string))
-	case map[interface{}]interface{}:
+	case map[string]interface{}: // JSON and TOML
+		return v
+	case map[interface{}]interface{}: // YAML
 		return v
 	}
+
+	jww.ERROR.Printf("GetParam(\"%s\"): Unknown type %s\n", key, reflect.TypeOf(v))
 	return nil
 }