make requests.request('HEAD',...) behave the same as requests.head(...)

Currently requests.head() behaves differently (doesn't follow redirects) than requests.request('HEAD', ...)

This is, I believe, undesirable behaviour, and also disagrees with the documentation (http://docs.python-requests.org/en/latest/user/quickstart/#redirection-and-history).

This also addresses the problem described in kennethreitz/grequests#45