if fields is [] return just _id

lib/mongo/collection.rb

@@ -85,7 +85,7 @@ module XGen
         #              http://www.mongodb.org/display/DOCS/How+to+do+Snapshotting+in+the+Mongo+Database
         def find(selector={}, options={})
           fields = options.delete(:fields)
-          fields = nil if fields && fields.empty?
+          fields = ["_id"] if fields && fields.empty?
           offset = options.delete(:offset) || 0
           limit = options.delete(:limit) || 0
           sort = options.delete(:sort)

test/test_collection.rb

@@ -124,6 +124,7 @@ class TestCollection < Test::Unit::TestCase
 
     assert @@test.find_one(nil, :fields => ["hello"]).include?("hello")
     assert !@@test.find_one(nil, :fields => ["foo"]).include?("hello")
+    assert_equal ["_id"], @@test.find_one(nil, :fields => []).keys()
 
     assert_equal nil, @@test.find_one("hello" => "foo")
     assert_equal nil, @@test.find_one(OrderedHash["hello", "foo"])