mediawiki.api: Fix API postWithToken method

Referred to "this" instead of "api" ("this" here refers to the Deferred object).
Fixes "Uncaught TypeError: Cannot read property 'ajax' of undefined".

Change-Id: I78ba47a614512f6218e23d03e7c688e2c9efbe45
(cherry picked from commit 93fc25c4a84916316c75c2b9b4065ee236a501df)