Ref T13575. Since PHP builtin webserver support was added, the pathway for parsing request parameters became more complex. We now rebuild "$_REQUEST" later, and this rebuild will destroy any mutations made to it here, so the assignment to __path__ is lost.

Instead of "validating" the request path, make this method "read" the request path and store it explicitly, so it will survive any later request mutations.

• Submitted any POST form while running Phabricator under the builtin PHP webserver. Old behavior was an error when accessing __path__; new behavior is a working application.
• Loaded normal pages, etc.