Use default DB in InfluxDBClient.write.

[bgusach]

Centralize default parameters in a function, and reuse these defaults in
InfluxDBClient._write_points and .query.

[patricktokeeffe]

A better approach would be to collect params inside write(), as was done for write_points() in 7fae65f

Also, write() should get a new database=None argument (like write_points has) that documents the use of connection's default database.