GitLab

* Staticcheck fixes

This fixes all staticcheck warnings for this library.

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".

We inserted these while transitioning to the new API but we've never hit one.

This time, we completely separate error encoding and let the emitter deal with it. Basically:

1. Some transports won't even use the same channel.
2. We don't *have* an error "encoder" for protobuf.

We now *finally* return the correct content type (json) when returning an error after passing `--enc=protobuf`.

This *also*:

1. Removes GetDecoder. We need to special case this every time anyways.
2. Allows transports to specify the "default" encoding type instead of assuming JSON.
3. Ensure that we never mistake a value for an error in the HTTP transport by converting to a cmdkit.Error *before* passing errors to `doPreamble`.

Only use custom encoders for values.

fixes #65

Also, remove SetEncoder. Always derive the correct encoder from the request.

- setting the error is moved into the preamble
- we call the preamble before the err!=nil branch because we send to the conn in there

- remove HandleError() because we now have CloseWithError
- remove Header interface
- remove all Head() methods
- fix locking in chan.go
- use EmitChan in ResponseEmitters
- introduce cmds.ErrClosedEmitter
- fix tests (some still expected errors to be emitted/marshaled)

**Returning errors/calling:**

Instead of having a dance where we:
  * Close the response emitter immediately if the command succeeds.
  * Return an error, call SetError, and then close the response emitter if it
    fails.

Just set the error from within the `Call` command and always close the response
emitter.

**Doner**

Get rid of the Done channel/doner interface from the response emitter. We close
the response emitter after finishing the Call *anyways* so this channel isn't
necessary (anymore, at least).