Fix for object data

I have done few test more, and noticed that if data in ajax is type of object and not string, this "content-length" header needs to be deleted.

Again I am not sure why, but I guess that if content-length is forwarded to request, request lib compares this content-length with new one (the one calculated by it self-request) which maybe is not the same on the end because this content-length if data is type of object is calculated differently in first place, I guess.
Previous "if" wasn't good because content-type has encoding within, so I've changed string comparison to substring match.