[postprocessor:metadata] assume 'mode: custom' when format is set

{"name": "metadata", "format": "foobar"}
will now implicitly use mode:custom and no longer mode:json like before

test/test_postprocessor.py

@@ -267,6 +267,7 @@ class MetadataTest(BasePostprocessorTest):
         test({"mode": "custom", "content-format": "{foo}\n{missing}\n"})
         test({"mode": "custom", "content-format": ["{foo}", "{missing}"]})
         test({"mode": "custom", "format": "{foo}\n{missing}\n"})
+        test({"format": "{foo}\n{missing}\n"})
 
     def test_metadata_extfmt(self):
         pp = self._create({