Add check for database name validity against the following description, from the connection string specification, of disallowed characters:

The following characters MUST NOT appear in the database name, once it has been decoded: slash, backslash, space, double-quote, or dollar sign.

Because ConnectionString will delegate to MongoNamespace#checkDatabaseNameValidity for this check, and MongoNamespace has to support namespace strings like "$external.$cmd", the Java driver will not prohibit the dollar sign character at this time.