Tested with a 3 node replica set using auth.

1. Dump everything using admin credentials works:
   ./mongodump --username admin --password mypassword --out dumpfiles
   connected to: 127.0.0.1
   all dbs
   DATABASE: foo to dumpfiles/foo
   foo.things to dumpfiles/foo/things.bson
   10000 objects
   foo.system.indexes to dumpfiles/foo/system.indexes.bson
   2 objects
   foo.system.profile to dumpfiles/foo/system.profile.bson
   60 objects
   foo.system.users to dumpfiles/foo/system.users.bson
   2 objects
   DATABASE: admin to dumpfiles/admin
   admin.system.users to dumpfiles/admin/system.users.bson
   1 objects
   admin.system.indexes to dumpfiles/admin/system.indexes.bson
   1 objects

1. Dumping a specific database using admin credentials fails
   ./mongodump --username admin --password mypassword --db foo --out dumpfiles
   connected to: 127.0.0.1
   Tue Sep 27 11:45:38 DBClientCursor::init call() failed
   assertion: 10276 DBClientBase::findN: transport error: 127.0.0.1 query: { authenticate: 1, nonce: "3112e50c77ef2be", user: "admin", key: "173615f515dc85297a54cb81933675d9" }

1. Dumping a specific database with user credentials works
   ./mongodump --username foouser --password foopassword --db foo --out dumpfiles
   connected to: 127.0.0.1
   DATABASE: foo to dumpfiles/foo
   foo.things to dumpfiles/foo/things.bson
   10000 objects
   foo.system.indexes to dumpfiles/foo/system.indexes.bson
   2 objects
   foo.system.profile to dumpfiles/foo/system.profile.bson
   49 objects
   foo.system.users to dumpfiles/foo/system.users.bson
   2 objects