Faster PNG striping

I found that when the image is really big the ChunkyPNG gem is slow to
encode/decode the PNG pixels. As we only want to strip the metadata we
can work in the Datastream that doesn't transform the pixels.

With my test image this method went from 10s to 0.01s.

lib/sprockets/svg.rb

@@ -18,9 +18,15 @@ def self.convert(svg_blob)
     end
 
     def strip_png_metadata(png_blob)
-      image = ChunkyPNG::Image.from_blob(png_blob)
-      USELESS_PNG_METADATA.each(&image.metadata.method(:delete))
-      image.to_blob
+      image = ChunkyPNG::Datastream.from_blob(png_blob)
+
+      image.other_chunks.reject! do |chunk|
+        chunk.respond_to?(:keyword) && USELESS_PNG_METADATA.include?(chunk.keyword)
+      end
+
+      str = StringIO.new
+      image.write(str)
+      str.string
     end
 
     def install(assets)

spec/rails_spec.rb

@@ -52,8 +52,6 @@
       png_metadata = ChunkyPNG::Image.from_file(png_path.to_s).metadata
       expect(png_metadata).to be_empty
 
-      # Metadata usually contains some timestamps, so sleeping is the best way to make sure they are all stripped
-      sleep 1
       expect(png_fingerprint).to be == Digest::MD5.hexdigest(Sprockets::Svg.convert(svg_path.read))
 
       expect(manifest['files'][svg_name]).to be_present