Fix remote_x_url= raise undefined method `[]' for nil:NilClass (NoMethodError)

[fukayatsu]

Fix remote_x_url= when the response header includes content-disposition: inline;filename=""

Please see #2411 (comment) for detail.

Maybe related to #2415 also.

[fukayatsu]

I see Travis CI is falling but it seems not related to this PR.
I'll rebase this branch if CI fixed :)

I could pass remote_file_spec.rb locally.

$ bundle exec rspec spec/downloader/remote_file_spec.rb
.......

Finished in 0.00647 seconds (files took 0.70994 seconds to load)
7 examples, 0 failures

[mshibuya]

Awesome, thanks!
I'll work on the CI later.

[fukayatsu]

Thank you for your hard work!