Specifying extensions as null in POST request causes INVALID_GRAPHQL_REQUEST error

[lorf]

Describe the bug

When making an HTTP POST request with extensions field set to null Apollo Router returns INVALID_GRAPHQL_REQUEST error:

$ curl -H 'content-type: application/json' -d '{"query":"{__typename}","extensions":null}' http://127.0.0.1:4000/
{"errors":[{"message":"Invalid GraphQL request","extensions":{"details":"failed to deserialize the request body into JSON: invalid type: null, expected a map at line 1 column 41","code":"INVALID_GRAPHQL_REQUEST"}}]}

However "GraphQL over HTTP" spec draft says that the extensions field is an optional request parameter (https://graphql.github.io/graphql-over-http/draft/#sec-Request-Parameters ). It also says (https://graphql.github.io/graphql-over-http/draft/#note-22957 ):

Note: Specifying null for optional request parameters is equivalent to not specifying them at all.

To Reproduce
When specifying extensions field in POST request as null, the following error is produced:

$ curl -H 'content-type: application/json' -d '{"query":"{__typename}","extensions":null}' http://127.0.0.1:4000/
{"errors":[{"message":"Invalid GraphQL request","extensions":{"details":"failed to deserialize the request body into JSON: invalid type: null, expected a map at line 1 column 41","code":"INVALID_GRAPHQL_REQUEST"}}]}

However, omitting extensions field works as expected:

$ curl -H 'content-type: application/json' -d '{"query":"{__typename}"}' http://127.0.0.1:4000/
{"data":{"__typename":"Query"}}

Expected behavior
Apollo Router should process POST requests with extensions field set to null as if there was no extensions field specified.

Output
Please, see To reproduce section.

Desktop (please complete the following information):

• OS: Ubuntu
• Version: 18.04

Additional context
Setting null_is_default = true here seems to fix the issue - requests with extensions=null start working as expected, but this was not thoroughly tested.

$ git diff
diff --git a/apollo-router/src/request.rs b/apollo-router/src/request.rs
index cbd3c3d7..dfe91bc7 100644
--- a/apollo-router/src/request.rs
+++ b/apollo-router/src/request.rs
@@ -281,7 +281,7 @@ impl<'data, 'de> DeserializeSeed<'de> for RequestFromBytesSeed<'data> {
                             }
                             let seed = serde_json_bytes::value::BytesSeed::new(self.0);
                             let value = map.next_value_seed(seed)?;
-                            let null_is_default = false;
+                            let null_is_default = true;
                             extensions = Some(as_object(value, null_is_default)?);
                         }
                         Field::Other => {