Issue 33

Fixes #33

This will make headers case insensitive. Right now headers set by the user are not normalised maybe we should do that @jnonsson?

```
http://0.0.0.0/> headers-set authorization 'Basic Zm9vOmJhcg=='
http://0.0.0.0/> headers-request
   User-Agent: htty/1.4.1
authorization: Basic Zm9vOmJhcg==
```
This could be ok, but…
```
http://foo:bar@0.0.0.0/> userinfo-set foz baz
http://foz:baz@0.0.0.0/> headers-request
    User-Agent: htty/1.4.1
Authorization:@ Basic Zm96OmJheg==
```

what do you think? @njonsson