fix signature bug to use RAW_PATH_INFO

This fixes signature creation to use RAW_PATH_INFO.

Swift3 could not create correct signature in case of
using escaped character(e.g. %2F, %2D) in PATH_INFO,
because env['PATH_INFO'] was decoded(unescaped) by
eventlet.wsgi before a request arrived at swift3.
It caused signature mismatch and authentication failure.

This change enables swift3 to create signature from
RAW_PATH_INFO and fixes that bug.

Note: This patch works well only in later version than
      eventlet 0.9.17, because older version does not
      have RAW_PATH_INFO variable.
      When using older version, swift3 works in the same
      way as ever(use req.path of swob).

Signed-off-by: Kota Tsuyuzaki <tsuyuzaki.kota@lab.ntt.co.jp>