Make verify_ld_signature more robust. Make Tombstone objects signable.

federation/entities/activitypub/ldsigning.py

@@ -99,6 +99,6 @@ class NormalizedDoubles(jsonld.JsonLdProcessor):
             item['@value'] = math.floor(value)
         obj = super()._object_to_rdf(item, issuer, triples, rdfDirection)
         # This is to address https://github.com/digitalbazaar/pyld/issues/175
-        if obj.get('datatype') == jsonld.XSD_DOUBLE:
+        if obj and obj.get('datatype') == jsonld.XSD_DOUBLE:
             obj['value'] = re.sub(r'(\d)0*E\+?(-)?0*(\d)', r'\1E\2\3', obj['value'])
         return obj

federation/entities/activitypub/models.py

@@ -1205,6 +1205,7 @@ class Retraction(Announce, base.Retraction):
 
 class Tombstone(Object, base.Retraction):
     target_id = fields.Id()
+    signable = True
 
     def to_as2(self):
         if not isinstance(self.activity, type): return None