mediawiki.api: Support assert parameter in getToken and postWithToken

Currently if you try to use assert=user with postWithToken the
request will fail with "token-missing" instead of "assertuserfailed"
because the assertion is not copied to getToken request.

Change-Id: Ibe48956adc6d2f171f88e6ef2bba8a39ae0127f2
Merge "mediawiki.api: Support assert parameter in getToken and postWithToken"