Can't force PyMongo to send w=1 write concern

By default, PyMongo correctly calls the "insert" command with no writeConcern, and calls getLastError with no "w" argument.

But PyMongo should include writeConcern of "w=1" with the insert command, or call getLastError with "w=1", if configured like this:

MongoClient('mongodb://localhost?w=1').db.collection.insert({})

Or if called like this:

collection.insert({}, w=1)

The purpose of sending w=1 is to override getLastErrorDefaults. Since getLastErrorDefaults are rarely, if ever, configured, this is a low-priority bug.