fixes JRuby issue with rubyzip

it looks like a .read is missing here, since in JRuby the "f" is still a Zip::InputStream.
Inside Nokogiri a the following exception is raised "Uncaught exception: must be kind_of String or respond to :to_io or :string", if the .read is missing.
With this change it works in MRI (tested with 2.1.2) and JRuby (tested with 1.7.13)