Make -colorspace argument optional

It will still default to 'RGB' but if you explicitly pass in
`save..., colorspace: nil` it will leave out the `-colorspace`
option and let imagemagick figure out the best colorspace on it's own.

We've seen some problems with some conversions, but this fixed the issue
for us.

Author: adamcrown

lib/grim/image_magick_processor.rb

@@ -36,7 +36,7 @@ def save(pdf, index, path, options)
       command << "-antialias"
       command << "-render"
       command << "-quality #{quality}"
-      command << "-colorspace #{colorspace}"
+      command << "-colorspace #{colorspace}" unless colorspace.nil?
       command << "-interlace none"
       command << "-density #{density}"
       command << "#{Shellwords.shellescape(pdf.path)}[#{index}]"