Don't peek unless the object quacks like an IO

lib/dimensions/io.rb

@@ -40,7 +40,7 @@ def peek
       end
 
       def no_peeking?
-        @reader.width || closed? || pos != 0
+        @reader.width || (!respond_to?(:closed?) || closed?) || (!respond_to?(:pos) || pos != 0)
       end
   end
 end

test/test_io.rb

@@ -1,6 +1,8 @@
 require 'dimensions/test_case'
 
 class TestIO < Dimensions::TestCase
+  class NotIO; end
+
   def test_extends_io
     with_extended_file do |file|
       assert file.is_a?(Dimensions::IO)
@@ -31,6 +33,10 @@ def test_skips_peeking_if_offset
     end
   end
 
+  def test_skips_peeking_unless_quacks_right
+    assert_equal nil, Dimensions(NotIO.new).dimensions
+  end
+
   private
     def with_extended_file
       with_fixture("screenshot.png") do |file|