should use the right buffer size
Typo in doc for _mongoc_cluster_sendv.
Merge pull request #144 from ajdavis/mutated

Typo in doc for _mongoc_cluster_sendv.
CDRIVER-489: Use the provided username, if available for X.509 auth

Falls back to extracting the subject from the cert for backwards
compatibility
CDRIVER-492: Report WriteConcern error on writes

Renamed the field, and fixed the failure check.
There can only ever be one WriteConcernError, which is a bson object
containing the code (int), errInfo (object) and errmsg (cstring),
so there is no arrays to merge.