Add Content-Length header

When using servant, none of the end-points return a "Content-Length" header. Instead, all of the response bodies are returned streaming.

Since servant endpoints generally always return a full object (JSON object, HTML object, etc...), I believe the size of the response is known, and the "Content-Length" header can be set.

The missing "Content-Length" header causes degraded performance when using [Network.Wai.Middleware.Gzip](https://hackage.haskell.org/package/wai-extra) middleware. This is because that middleware checks the "Content-Length" header in order to skip compression on small response bodies (less than 860 bytes). The source is here: and here is the reasoning:

Servant is typically used for JSON APIs, so many responses in real projects will be small JSON payloads (less than 860 bytes).