Force ascii-8bit encoding when creating tempfile

As I understand it, the default in application.rb for Rails 3+ is:

  config.encoding = "utf-8"

The content that comes back from docraptor is encoded ascii-8bit. If the
default Rails encoding is not changed, then the your application will
try to convert the content, and the gem frequently throws an error
similar to:

  "\xE2" from ASCII-9BIT to UTF-8

This error is originating with the creation of a tempfile.

This fix adds an option to the Tempfile.open call to force encoding to
ascii-8bit.