lowercase patch method not permitted on window fetch #305

There seems to be an error related to the update resource of google AIP.

As specified in AIP, update resource should be sent by patch method, and gax-nodejs is also supposed to send request to patch.
However, for window fetch https://fetch.spec.whatwg.org/#methods, as you can see here, patch is not allowed as the default specification.

So, I think it would be better to change it to uppercase ‘PATCH’ instead of lowercase ‘patch’, what do you think?

In fact, when I use this library in production it passes 400 Bad Requests. And when I fix the build to use the method with the name "PATCH" in local, got no errors.

Environment details

• OS: macOS 12.4
• Node.js version: v18.7.0
• npm version: v8.15.0
• google-gax version: 2.30.4

Steps to reproduce

1. create update resource service

// Updates an existing Resource And Returns the updated Resource.
  rpc UpdateResource(UpdateResourceRequest) returns (Resource) {
    option (google.api.http) = {
      patch: "/api/v1/{resource.name=resources/*}"
      body: "resource"
    };
    option (google.api.method_signature) = "resource,update_mask";
  }

2. build it with google-gax
3. use the service

const resourceService = new ResourceServiceClient({
    auth: { getRequestHeaders: async () => ({ authorization: "Bearer abcd" }) },
    fallback: 'rest',
    protocol: 'https',
    apiEndpoint: 'api.end.point',
    port: 443,
});
await resourceService.updateResource(request);