Use customHeaders.Authorization in httpAuth module

I'm not sure if this is by design in phantomjs, but believe this is required in places where basic auth is optional because phantomjs will only send the headers when it sees a 401. See http://stackoverflow.com/a/15940361/203891 and https://github.com/ariya/phantomjs/issues/10590

For some reason adding the header (page.customHeaders.Authorization = ) directly doesn't work. Probably because of the way it is defined in https://github.com/ariya/phantomjs/blob/48fabe06463460d2fb7026d6df9783216e26265c/src/webpage.h#L71 

The page.customHeaders object should be extended in case there's headers already set at this point, but I didn't know the best way to do that.
Can you test if this would work in SlimerJS (``--gecko``) as well?