count() is 0 for non-existant collections, not an error

lib/mongo/db.rb

@@ -390,6 +390,7 @@ module XGen
           oh[:query] = selector || {}
           doc = db_command(oh)
           return doc['n'].to_i if ok?(doc)
+          return 0 if doc['errmsg'] == "ns missing"
           raise "Error with count command: #{doc.inspect}"
         end
 

tests/test_db_api.rb

@@ -56,6 +56,11 @@ class DBAPITest < Test::Unit::TestCase
     assert docs.detect { |row| row['b'] == 3 }
   end
 
+  def test_count_on_nonexisting
+    @@db.drop_collection('foo')
+    assert_equal 0, @@db.collection('foo').count()
+  end
+
   def test_find_simple
     @r2 = @@coll.insert('a' => 2)
     @r3 = @@coll.insert('b' => 3)