Messing up Transfer-Encoding and Content-Length headers

Explicitly set "Transfer-Encoding: chunked" might lead to a chunked response being sent alongside with the Content-Length header.

```clojure
(require '[aleph.http :as http])
(http/start-server
(fn [_] {:status 200
:headers {"transfer-encoding" "chunked"}
:body (java.io.File. "/path/to/file")})
{:port 2018 :compression? true})
```

The raw response looks like

```HTTP/1.1 200 OK
Transfer-Encoding: chunked
Server: Aleph/0.4.6
Date: Sun, 30 Dec 2018 02:36:44 GMT
Connection: Keep-Alive
content-length: 637315352

2000

```

`:compression?` set to `true` [makes](https://github.com/ztellman/aleph/blob/be1ee04c130660ce69ee7c6603132b36be79ec1f/src/aleph/http/server.clj#L433-L436) the server to include `ChunkedWriteHandler` onto the pipeline, so the file will be sent using `send-chunked-body` which implicitly [adds](https://github.com/ztellman/aleph/blob/be1ee04c130660ce69ee7c6603132b36be79ec1f/src/aleph/http/core.clj#L330) `Content-Length` header. `HttpResponseEncoder` encodes chunks produced by `HttpChunkInput` with chunked coding rules (as the presence of the appropriate header dictates to do it). It doesn't cause any issues for a lot of clients, but following [RFC2616](https://www.w3.org/Protocols/rfc2616/rfc2616.txt)

> 3.If a Content-Length header field (section 14.13) is present, its
> decimal value in OCTETs represents both the entity-length and the
> transfer-length. The Content-Length header field MUST NOT be sent
> if these two lengths are different (i.e., if a Transfer-Encoding
> header field is present).

I think we need to check if this header is set explicitly before setting Content-Length. Unfortunately It's not very intuitive way of managing encodings, but I feel like removing explicitly set header would be even more confusing than not adding a new one. Also, we probably need to ensure the header is set when sending [chunked body](https://github.com/ztellman/aleph/blob/be1ee04c130660ce69ee7c6603132b36be79ec1f/src/aleph/http/core.clj#L334-L336).