Make session persist between calls to doApiRequest

Make session persist between calls to doApiRequest

Make sure the global session data in $wgRequest is used for doApiRequest
per default, and return it's content among with the request's results.

Previously, an empty session was used per default, and the local context's
session data would get out of sync with $wgRequest.

This change allows for the following assumptions to hold in test cases:

* within the same function, changes to the session made by one api call
  will be visible to subsequent api calls.

* the session data returned by doApiRequest is the actual status of the
  session as manipulated by the api call. This session data can be passed
  to subsequent api calls.

Note that the session data is still reset for every call to a test
function.

Change-Id: Ia20cf0ccfcdca736dd5da3444b14fbdd1c5def46