Add Content-length to simplewebcontrol example to increase compatibility.

[jwurzer]

I had the problem that the HTTPRequest node (a HTTP client) from the godot engine doesn't handle the JSON content from the simplewebcontrol server example (examples/simplewebcontrol.py). The JSON content was always discarded. I have to add the Content-length property to the header at the simplewebcontrol example. I think this changes could also increase the compatibility of other HTTP clients and browsers. The pull request #42 adds the necessary changes.