Headers for download by url

[mendab1e]

Hi. I use CarrierWave in my project, where I need to download some images using authorization token in header. I haven't found support of sending custom headers in request, so I've implement it in this pull request.

Here is the example of usage:

class Content
  extend CarrierWave::Mount
  mount_uploader :image, ImageUploader
end

c = Content.new
c.remote_image_request_header = {'Authorization' => 'Bearer QWERTY'}
c.remote_image_url = 'http://example.com/1.jpg'

[mshibuya]

Using reverse_merge can be useful when a user want to set specific User-Agent.

[mendab1e]

@mshibuya Good point. I've pushed new commit with reverse_merge.

[mshibuya]

Nice work, thanks!