Set the contentLength on the AtmosphereRequest.

Hi JeanFrancois

the ```contentLength``` is not correctly reported from the ```AtmosphereRequest``` as ```createAtmosphereRequest``` in the ```NettyAtmosphereHandler``` has no means of setting it (it is always 0, stemming from ```NoOpsRequest ```, I think). There is no ```HttpServletRequest``` object it could set on the ```Builder.request``` field. I would thus suggest to :

- introduce a ```contentLength``` field in the ```Builder``` (see my [complementing pull request in Atmosphere](https://github.com/Atmosphere/atmosphere/pull/999)) , with a defensive extension to the ```getContentLength``` method of ```AtmosphereRequest```.
- and change ```createAtmosphereRequest``` to retrieve the contentLength from the headers and set the newly introduced ```Builder``` field for ```contentLength```.

This is only a proposal, maybe there is a more elegant way to have the  ```AtmosphereRequest ``` reflect the proper content length.

Cheers, Christian.