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* fix: allow requests from electron renderer

The Electron Renderer process runs in an embedded  browser window so
has access to browser-native `fetch`.  If this is used by
`ipfs-http-client` to make requests against the HTTP API, the
`User-Agent` header is set to a value that looks similar to a browser
but no `Origin` or `Referer` headers are sent.

The change here is to relax the user agent check to allow through
requests from clients with `'Electron'` in their user agents.

* Staticcheck fixes

This fixes all staticcheck warnings for this library.

Addresses browsers being able to POST without control due to things like
https://bugzilla.mozilla.org/show_bug.cgi?id=429594

Allowing methods isn't as simple as just allowing/disallowing them because
different methods do different things.

* HEAD: should be allowed everywhere GET is allowed.
* OPTIONS:
  * When a CORS request is made, this will be handled by the CORS library.
  * Otherewise, we need to return the allowed methods.
* POST: always allowed.
* Everything else: always denied.

Changing HandledMethods to a simple AllowGet makes it easier to "do the right
thing".

This Handler option is used it to allow/deny request by their method.

The API is an RPC API so it normally should only work with PUT, but we also
use a read-only, GET-based partial API that runs with the gateway.

This commit makes the actual allowed (or handled) methods configurable.

A test is added, and test facilities improved to be able to set/modify
this option.

fixes #42