Cannot serialize transaction when using valid type

[FrederikBolding]

Describe the bug
Calling serialize on a Transaction with type equals null, results in an error being thrown because checkProperties doesn't expect type.

Reproduction steps

const transaction = {
      to: '0x4bbeEB066eD09B7AEd07bF39EEe0460DFa261520',
      value: BigNumber.from('0x0'),
      data: '0x',
      chainId: 1,
      gasLimit: BigNumber.from('0x5208'),
      gasPrice: BigNumber.from('0x1'),
      nonce: 1,
      r: '0x7e833413ead52b8c538001b12ab5a85bac88db0b34b61251bb0fc81573ca093f',
      s: '0x49634f1e439e3760265888434a2f9782928362412030db1429458ddc9dcee995',
      v: 37,
      type: null
    };
 const { r, s, v, ...unsignedTransaction } = transaction;
 const serializedTransaction = serialize(unsignedTransaction, { r, s, v });

Running the above code results in the following error:

invalid object key - type (argument="transaction:type", value={"to":"0x4bbeEB066eD09B7AEd07bF39EEe0460DFa261520","value":{"type":"BigNumber","hex":"0x00"},"data":"0x","chainId":1,"gasLimit":{"type":"BigNumber","hex":"0x5208"},"gasPrice":{"type":"BigNumber","hex":"0x01"},"nonce":1,"type":null}, code=INVALID_ARGUMENT, version=properties/5.1.0)

[ricmoo]

This has been fixed in 5.4.0. Try it out and let me know if you still have any issues. :)