fix: properly marshall URL fields to JSON

Marshalling to JSON is used in FFI bindings to pass data around.

Author: bfabio

marshallers.go

@@ -1,6 +1,7 @@
 package publiccode
 
 import (
+	"encoding/json"
 	"net/url"
 )
 
@@ -27,6 +28,13 @@ func (u *URL) MarshalYAML() (interface{}, error) {
 	return u.String(), nil
 }
 
+func (u *URL) MarshalJSON() ([]byte, error) {
+	if u == nil {
+		return []byte(`null`), nil
+	}
+	return json.Marshal(u.String())
+}
+
 func (u URL) String() string {
 	return (*url.URL)(&u).String()
 }