Allow updating gandi rrset ttl (#512)

Previously, a rrset ttl could be set only when creating a new record,
whereas with this change, now a new ttl value can be specified.

lexicon/providers/gandi.py

@@ -98,6 +98,8 @@ def _create_record(self, rtype, name, content):
                 self.domain_id, self._relative_name(name), rtype)
             if current_values:
                 record = {'rrset_values': current_values + [content]}
+                if self._get_lexicon_option('ttl'):
+                    record['rrset_ttl'] = self._get_lexicon_option('ttl')
                 self._put(url, record)
             else:
                 record = {'rrset_values': [content]}