Correctly calculate Content-Length as a byte size
Correctly calculate Content-Length as a byte size

The HTTP server currently is calculating Content-Length as a naive
character count, but the header is supposed to be the byte size of the
response contents. In the case of "Hello World", the two are equivalent,
but, if the response contained special characters, the Content-Length
would be incorrect:

```swift
print("moose".characters.count) // 5
print("møøse".characters.count) // 5
print("møøse".utf8.count) // 7
```

For semantic reasons, and for people who are copying the source to serve
their own custom responses, we should provide the correct byte length.

Signed-off-by: David Celis <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@davidcel.is>

huytd/swift-http,camelCaseD/swift-http,huytd/swift-http,camelCaseD/swift-http