Add support to rotate the image according to the exif information

The ImageMagick option "-auto-orient" uses the "Orientation" tag
in the exif metadata to rotate the image into the right position
for clients which don't support rotating images on the fly.

This fixes problems with image uploads from misc cameras, for
example the Apple iPhone.

lib/dragonfly/image_magick/processor.rb

@@ -26,6 +26,10 @@ def resize(temp_object, geometry)
         convert(temp_object, "-resize \"#{geometry}\"")
       end
 
+      def auto_orient(temp_object)
+        convert(temp_object, "-auto-orient")
+      end
+
       def crop(temp_object, opts={})
         width   = opts[:width]
         height  = opts[:height]

spec/dragonfly/image_magick/processor_spec.rb

@@ -237,6 +237,17 @@
     end
   end
 
+  describe "auto-orient" do
+    it "should rotate an image according to exif information" do
+      @image = Dragonfly::TempObject.new(SAMPLES_DIR.join('beach.jpg'))
+      @image.should have_width(355)
+      @image.should have_height(280)
+      image = @processor.auto_orient(@image)
+      image.should have_width(280)
+      image.should have_height(355)
+    end
+  end
+
   describe "flip" do
     it "should flip the image, leaving the same dimensions" do
       image = @processor.flip(@image)