eval compatible repr for `Request`

After this change:
  In [1]: from requests import Request

  In [2]: eval(repr(Request('GET', 'http://x.org')))
  Out[2]: Request(method='GET', url='http://x.org')

  In [3]: eval(repr(Request('GET', 'http://x.org', auth=('hello', 'world'))))
  Out[3]: Request(method='GET', url='http://x.org', auth=('hello', 'world'))