Introduce explicit contentLength property on the AtmosphereRequest.Builder

Hi JeanFrancois

[Complementing my other pull request in Nettosphere] (https://github.com/Atmosphere/nettosphere/pull/35) I would suggest to introduce an explicit contentLength property in the ```AtmosphereRequest.Builder``` (, with a defensive extension to the ```getContentLength()``` method of ```AtmosphereRequest```).

Cheers, Christian.