Nightly breaks

This fixes the nightly breaks. Specifically:

- cast must take an explicit type.
- the boxxed ResponseWriter must be dereferenced to take an &mut.

This leaves warnings in place to this effect:

```
src/http/buffer.rs:125:43: 125:88 warning: use of deprecated item: use fmt::radix, #[warn(deprecated)] on by default
src/http/buffer.rs:125                 let s = format!("{}\r\n", (self.write_len + buf.len()).to_str_radix(16));
```