[couchbeam] allow 202 to be a valid answer, as by save_doc

Summary:
As all tests for bulk implementation were for couchdb, it seems, that
bigcouch sometimes answers with 202 for bulk methods too.

Test Plan: read, test with bigcouch

Reviewers: aschultz, vtarasen

Reviewed By: aschultz

CC: mkraemer, aschultz

Differential Revision: https://review.tpip.net/D1383