fix for latest rust compatibility

Or it refuses to build:

```
Could not execute process `rustc src/http/lib.rs --crate-type lib --out-dir /home/kstep/git/rusttut/adslbystat/target/deps -L /home/kstep/git/rusttut/adslbystat/target/deps -L /home/kstep/git/rusttut/adslbystat/target/deps` (status=101)
--- stderr
src/http/server/request.rs:269:19: 269:44 error: cannot index a value of type `&str`
src/http/server/request.rs:269         } else if request_uri.as_slice()[0] as char == '/' {
                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
```