Fixes for Swift 1.7.6

@cschwede, we're testing the middleware with Swift 1.7.6 and we're facing some issues.

The patch works for us but I don't think the implementation I've done is good enough, it's just a PoC.

In a nutshell, we're having two issues, I've added some comments to the patch.

* The first one is that the middleware raises an exception trying to split a tuple:

```
proxy-server Error: 'tuple' object has no attribute 'sp
lit': #012Traceback (most recent call last):#012  File "/usr/lib/python2.7/dist-packages/swift/commo
n/middleware/catch_errors.py", line 36, in handle_request#012    resp = self._app_call(env)#012  Fil
e "/usr/lib/python2.7/dist-packages/swift/common/wsgi.py", line 270, in _app_call#012    resp = self
.app(env, self._start_response)#012  File "/usr/lib/python2.7/dist-packages/swift/common/middleware/
bvox_lbnode.py", line 21, in __call__#012    return self.app(environ, custom_start_response)#012  Fi
le "/usr/lib/python2.7/dist-packages/swift/common/middleware/healthcheck.py", line 57, in __call__#0
12    return self.app(env, start_response)#012  File "/usr/lib/python2.7/dist-packages/swift/common/
middleware/memcache.py", line 65, in __call__#012    return self.app(env, start_response)#012  File 
"/usr/lib/python2.7/dist-packages/swift/common/middleware/ratelimit.py", line 232, in __call__#012  
  return self.app(env, start_response)#012  File "/usr/lib/python2.7/dist-packages/keystone/middlewa
re/auth_token.py", line 278, in __call__#012    return self.app(env, start_response)#012  File "/usr
/lib/python2.7/dist-packages/keystone/middleware/swift_auth.py", line 131, in __call__#012    return
 self.app(environ, start_response)#012  File "/usr/lib/python2.7/dist-packages/swift/common/middlewa
re/proxy_logging.py", line 188, in __call__#012    iterable = self.app(env, my_start_response)#012  
File "/usr/lib/python2.7/dist-packages/account_quotas/middleware.py", line 94, in __call__#012    re
seller = self.reseller_request(env)#012  File "/usr/lib/python2.7/dist-packages/account_quotas/middl
eware.py", line 77, in reseller_request#012    if user and (set(user.split(',')) & set(valid_groups)
):#012AttributeError: 'tuple' object has no attribute 'split' (txn: tx66b5e35abca942b7b3ba1f15a6bf65
ee)
```

* The second issue is that the limits don't have any effect because the HEAD request in get_quota is heading the container instead of the account, since the account string it receives has the container appended (AUTH_XXXXXXXX/container-name).

Please let me know what you think. Perhaps those issues are not in swift/master, I did not check against the master branch.

Thanks!