#2 Fix HTTP module error while trying to write response headers

Fixes the issue #2

Maybe it's not the best way to solve it, but I didn't make a deep review of the code.

According with your OO design, I created a public method called `writeHead` in the `Portal` parent class in order to verify if the response head was already written. This method are used from `Admin` and `Stubs` classes.

Hope it can be merged as soon as possible.

Thank you so much for the amazing work you done!!
Cheers