Use customHeaders because httpAuth

I 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.