Handle content-disposition headers with blank filenames

When an image response has a content-disposition header with a blank filename (e.g. `inline;filename=""`), parse the filename from the image url
IMO the required argument to `LinkPreview::Parser` should be a readable `double`:
```ruby
let(:config) { double }
let(:parser) { LinkPreview::Parser.new(config) }
```
it's not referenced in this method, but if it were the rspec error message would be more meaningful than a NPE