Followup for undefined index "bot", add bot param to MergeItems

Follow up for Ie0650eb

MergeItems::getAllowedParams (and parent class methods) allow
setting a list of params with a default. "bot" was missing in
MergeItems, which extends ApiWikibase directly. Various other api
classes extend ModifyEntity or others that have "bot" set.

I believe that was the issue and this is a nicer, more consistent
way to fix the issue.

(still odd that the test failure happened only sometimes, though)

Change-Id: I16c8ea57618498bf22e603de1dd54b339d3a8f8a
(cherry picked from commit e7fe524832862bbb43157c514286905252223c88)
Merge "Followup for undefined index "bot", add bot param to MergeItems" into mw1.23-wmf19