Fix ImageModel missing :data option key

[victorpolko]

This PR is intended to fix the issue #131 and may be a duplicate of #132 by @yholkamp (I noticed that PR after the fix), but it:

1. adds a test case that fails without the fix
2. doesn't mess with ImageModel#relationship_target, which is only used to retrieve the file extension

It also relates to issue #27 in a way that even after this fix there's no actual possibility to create a non-corrupted image from Base64 data:

# /lib/caracal/document.rb#L222
def render_media(zip)
  images = relationships.select { |r| r.relationship_type == :image }
  images.each do |rel|
    if rel.relationship_data.to_s.size > 0
      content = rel.relationship_data
    else
      content = open(rel.relationship_target).read
    end

    zip.put_next_entry("word/#{ rel.formatted_target }")
    zip.write(content)
  end
end

The read method that's used with the File object when no :data was supplied returns binary data, not Base64-encoded.
Similarly, if you do pass :data to the img method, it has to be binary, too, and you'll get nice and valid resulting image:

doc.img image_path,
  data: File.read(image_path), # !! not Base64.encode64(File.read(image_path))
  width: 100,
  height: 100

With all that said, I guess that caracal-example has to be updated:

# app/views/examples/show.docx.caracal#L22
data = Base64.encode64(File.read('public/plia-login.png'))
docx.img 'public/plia-login.png', data: data, width: 400, height: 124, align: :right
# => 
data = File.read('public/plia-login.png')
docx.img 'public/plia-login.png', data: data, width: 400, height: 124, align: :right

I don't mind taking care of it after this PR is merged.