prepare_accept_content() now raises SerializerNotInstalled instead of KeyError (#1343)

* Fix broken nested() after #1320

* prepare_accept_content now raises SerializerNotInstalled when wrong serializer alias is passed

kombu/serialization.py

@@ -445,7 +445,17 @@ for ep, args in entrypoints('kombu.serializers'):  # pragma: no cover
 
 
 def prepare_accept_content(content_types, name_to_type=None):
+    """Replace aliases of content_types with full names from registry.
+
+    Raises:
+        SerializerNotInstalled: If the serialization method
+            requested is not available.
+    """
     name_to_type = registry.name_to_type if not name_to_type else name_to_type
     if content_types is not None:
-        return {n if '/' in n else name_to_type[n] for n in content_types}
+        try:
+            return {n if '/' in n else name_to_type[n] for n in content_types}
+        except KeyError as e:
+            raise SerializerNotInstalled(
+                f'No encoder/decoder installed for {e.args[0]}')
     return content_types

t/unit/test_serialization.py

@@ -13,7 +13,7 @@ from kombu.serialization import (
     raw_encode, register_yaml, register_msgpack,
     dumps, loads, pickle, pickle_protocol,
     unregister, register_pickle, enable_insecure_serializers,
-    disable_insecure_serializers,
+    disable_insecure_serializers, prepare_accept_content
 )
 from kombu.utils.encoding import str_to_bytes
 
@@ -307,3 +307,12 @@ class test_Serialization:
         register_msgpack()
         with pytest.raises(SerializerNotInstalled):
             loads('foo', 'application/x-msgpack', 'utf-8')
+
+    def test_prepare_accept_content(self):
+        assert {'application/json'} == prepare_accept_content(['json'])
+        assert {'application/json'} == prepare_accept_content(
+            ['application/json'])
+
+    def test_prepare_accept_content_bad_serializer(self):
+        with pytest.raises(SerializerNotInstalled):
+            prepare_accept_content(['bad_serializer'])