Don't ignore writeConcern in BSONCollection.save

The "save" helper, accepting a GetLastError as a parameter, invoked the
underlying GenericCollection.update and .insert without a writeConcern.
The default for the argument (GetLastError()) would then be used,
effectively ignoring the user provided writeConcern.
Merge pull request #188 from utaal/fix-writeconcern

Don't ignore writeConcern in BSONCollection.save