Fix _forward secret key matching expects standard router matching

[Vinai]

Background:
When you are in admin and using secret keys, the key in the request
has to match up with the controller + action + salt. If not you are
bounced to the dashboard. When you call _forward to get sent to a new
URL, the new dispatch loop call checks the secret key a second time.
The 'problem' at this stage is the secret key doesn't match up with
the controller and/or action portion. The 'fix' that currently is in
place is to grab the the original PATH_INFO from the request, split
that on '/', grab indexes 1 and 2, and use those as the
controller and action.
Keep in mind, this only happens if the controller and/or action are
not passed directly to the getSecretKey call. Additionally, if
either of these is empty then it has a second try of getting the
controller or action name stored in the request object directly.

Ok, that's the background. Here's the 'bug':
If you have code in place that handles routing differently than the
standard routers, the getSecretKey method erroneously making
assumptions about the translation of a PATH_INFO string into a
module/controller/action array. And, as luck would have it, the
request object has a MUCH better method of working around the issue.
The is a getBeforeForwardInfo method that can give the original
request module/controller/action that was made. That info would
correspond to what getSecretKey expects.

This patch was also submitted as MCACE-144 to the Magento 1.6.2.0
MCA-CE contributor repository in Mage_Adminhtml_Model_Url.

Thanks to Lee Saferite for this one!

[magento-team]

@Vinai
Thank you for the contribution, we have applied it as a patch. In addition, the changes have been covered with tests.
The changes will be rolled out in next update.

[Vinai]

Thank you!