CSHARP-554 - Implemented CopyDatabase on MongoServer

This is a relatively simple update.

1) Fixed a compilation issue in UpdateBuilder that was breaking the build. Updated references to "document" to the instance-level "_document" object in the Pull method.

2) Implemented CSHARP-554 - CopyDatabase in MongoServer. Did not include a fromHost parameter for now, as I figured we could add an overload in the future. To do that, we'd need a way to handle authentication on the host server of the original database. I added a test to the MongoServerTests.