NONE and NOASSERTION for 'no-license' and 'other'

https://spdx.org/spdx-specification-21-web-version#h.ihv636

lib/licensee/license.rb

@@ -117,6 +117,12 @@ module Licensee
       @meta ||= LicenseMeta.from_yaml(yaml)
     end
 
+    def spdx_id
+      return meta.spdx_id if meta.spdx_id
+      return 'NOASSERTION' if key == 'other'
+      return 'NONE' if key == 'no-license'
+    end
+
     # Returns the human-readable license name
     def name
       title ? title : key.capitalize