Add if-match (and other optional headers) to create object

[emmaling27]

It would be useful for optional headers (especially if-match) to be implemented so that logic can be on the server-side. Ideally, all optional headers should be accessible in this library to support full functionality of Google Cloud's JSON API. I just happen to need if-match and if-none-match for my use case.

[ThouCheese]

Sure this seems like a reasonable enhancement. Two issues come to mind about implementing this, namely that it requires a breaking change to almost every function in the library if we want to do something like Bucket::read("mybucket", hdrs). Secondly, to make this safe to use for the end user, it requires that for each function that accepts headers, a struct is defined that contains the headers (and query parameters) that are applicable to that endpoint.

[rrooij]

I would love this as well as I need if-match.

@ThouCheese It won't have to be a breaking change. We can also provide new functions like Bucket::read_with_headers and the like. We don't have to break the existing API for this.

[ThouCheese]

Sure but the downside would then be that we have to duplicate almost all functions: Bucket::create_with_headers, Bucket::update_with_headers, ... Maybe that we can create a function like Client::set_headers, with the new 0.9 release however.

[pjenvey]

If you're breaking the API at some point I recommend using the builder pattern for calls. It takes more effort to implement but is more resilient in the long term, allowing the API to expand in the future without breaking the contract.