Fails basic Hello World with Download
```nim
import jester

routes:
  get "/":
    resp "<a href='sample.zip'> Test Download </a>"
```

[OK.zip](https://github.com/dom96/jester/files/2568872/OK.zip)

[FAIL.zip](https://github.com/dom96/jester/files/2568875/FAIL.zip)

1. Create a `./public/` folder.
2. Put ZIP files on `./public/` folder.
3. Rename `FAIL.zip` to `sample.zip` to Fail.
4. Rename `OK.zip` to `sample.zip` to Success.
5. Run the Nim code provided `nim c -r file.nim`.
6. Visit http://localhost:5000/ and click on *Test Download*

Fails on Stable and Devel.

**Extra Info:**

<details>

```
$ nimble dump jester
name: "jester"
version: "0.4.1"
author: "Dominik Picheta"
desc: "A sinatra-like web framework for Nim."
license: "MIT"
skipDirs: "tests"
skipFiles: "todo.markdown"
skipExt: ""
installDirs: "jester"
installFiles: "jester.nim"
installExt: ""
requires: "nim >= 0.18.1, httpbeast >= 0.2.0, asynctools any version"
bin: ""
binDir: ""
srcDir: ""
backend: "c"

$ nimble --version
nimble v0.9.0 compiled at 2018-09-27 14:45:31
git hash: couldn't determine git hash

$ nim --version
Nim Compiler Version 0.19.0 [Linux: amd64]
Compiled at 2018-09-26
Copyright (c) 2006-2018 by Andreas Rumpf

git hash: f6c5c636bb1a1f4e1301ae0ba5a8afecef439132
active boot switches: -d:release

$ choosenim --version
choosenim v0.3.2 (2018-02-28 14:12:37) [linux/amd64]

juan@x ~  $ uname -a
Linux x 4.18.16-arch1-1-ARCH #1 SMP PREEMPT Sat Oct 20 22:06:45 UTC 2018 x86_64 GNU/Linux
```

Firefox Error:

```
Corrupted Content Error

The site at http://localhost:5000/sample.zip has experienced a network protocol violation that cannot be repaired.
The page you are trying to view cannot be shown because an error in the data transmission was detected.
```

Chromium Error:

```
localhost sent an invalid response.
ERR_RESPONSE_HEADERS_MULTIPLE_CONTENT_LENGTH
```

Fails with `127.0.0.` and `localhost` on the URL too.

Fails with URL starting with `/` too.

I think is something related to content size lengths.

</details>

:worried: 
