Minor improvements for request logging

With this change, even when `headers['Content-Length']` is not returned, no longer will we log:
```
I, [2014-11-11T17:01:10.138555 #33103]  INFO -- : Completed in 1179ms | 200 | - bytes
```

Instead, we will log:
```
I, [2014-11-11T17:21:18.903014 #34801]  INFO -- : Completed in 1209ms | 200 OK | 3156 bytes
```

Note that we now log the Status message instead of only the code. A minor thing, but helpful for support folks.
Is headers guaranteed to have canonical character case? i.e. if the browser sends "content-LENGTH" will Rack transform it for us? I'd double check.