[wip] Respond appropriately to HTTP OPTIONS method

[langston-barrett]

Enables development of JavaScript API clients that use CORS requests. Also, responding to OPTIONS requests with 200 OK is just a good baseline behavior.

This needs unit tests, but I wanted to get feedback on the way it's implemented first. I'm also happy to rebase & squash when the time comes.

Fixes #865
Obsoletes #1807

If this change is too large, we could also simply respond with 200 to any OPTIONS requests:

        var obj interface{}
        if req.Method == "OPTIONS" {
            obj, err = nil, nil
        } else { // Invoke the handler
            obj, err = handler(resp, req)
        }

For more information on the OPTIONS method, view the HTTP 1.1 RFC 2616, § 9.2. "OPTIONS".

[slackpad]

Hi @siddharthist thanks for the PR and sorry for the delay in looking at this. I'm thinking we should incorporate the listing of allowed methods into here. We could make a new registration function that turns this:

s.mux.HandleFunc("/v1/kv", s.wrap(s.AgentSelf))

Into something like this:

registerEndpoint("/v1/agent/self", "GET,PUT,DELETE", s.wrap(s.KVSEndpoint))

This way we can build the data structure one time alongside the registration of the endpoints. What do you think?

[langston-barrett]

@slackpad That makes a lot of sense. Most of the work on this PR was just finding out what endpoints respond to which methods, so I'm happy to redo the code :)

[langston-barrett]

@slackpad What are the supported methods on the /v1/txn endpoint?

[langston-barrett]

Closing until I have more time to work on it.