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.

Refer to https://github.com/ReactiveMongo/ReactiveMongo/blob/master/driver/src/main/scala/api/collections/genericcollection.scala#L196-L203 and https://github.com/ReactiveMongo/ReactiveMongo/blob/master/driver/src/main/scala/api/collections/genericcollection.scala#L166-L171