[fix] get doi_rewriters from settings - fixes #1245

searx/webapp.py

                     'safe_search': settings['search']['safe_search'],
                     'default_theme': settings['ui']['default_theme'],
                     'version': VERSION_STRING,
-                    'doi_resolvers': [r for r in search['doi_resolvers']],
+                    'doi_resolvers': [r for r in settings['doi_resolvers']],
                     'default_doi_resolver': settings['default_doi_resolver'],
                     })