requests have invalid cookie header

[alethiophile]

At https://github.com/theelous3/asks/blob/master/asks/request_object.py#L215, the Cookie header string is constructed from the cookies dictionary using the delimiter "; ". However, only the last character (rather than the last two characters) of the final string is trimmed off after the final iteration. This leaves an extraneous ; on the end of the Cookie header string; this is in violation of RFC 6265, and also breaks authentication on some servers I'm using.