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* Staticcheck fixes

This fixes all staticcheck warnings for this library.

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.

- 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)