util.RoundTrip(): use neighbouring UnmarshalJSON function

The test failures this had introduced before, from index_tests.go's
TestIndicesBuild, suggested that this is probably what we want.

Signed-off-by: Stephan Renatus <srenatus@chef.io>

util/json.go

@@ -65,5 +65,5 @@ func RoundTrip(x *interface{}) error {
 	if err != nil {
 		return err
 	}
-	return json.Unmarshal(bs, x)
+	return UnmarshalJSON(bs, x)
 }