minor: added grid store length accessor

lib/mongo/gridfs/grid_store.rb

@@ -54,6 +54,9 @@ module GridFS
     # Default is DEFAULT_CONTENT_TYPE
     attr_accessor :content_type
 
+    # Size of file in bytes
+    attr_reader :length
+
     attr_accessor :metadata
 
     attr_reader :files_id

test/test_grid_store.rb

@@ -71,6 +71,10 @@ class GridStoreTest < Test::Unit::TestCase
     assert_equal "hello", GridStore.read(@@db, 'foobar', 5)
   end
 
+  def test_access_length
+    assert_equal 13, GridStore.new(@@db, 'foobar').length
+  end
+
   # Also tests seek
   def test_read_with_offset
     assert_equal "world", GridStore.read(@@db, 'foobar', 5, 7)